Porous abrasive articles with agglomerated abrasives

ABSTRACT

A bonded abrasive tool, having a structure permeable to fluid flow, comprises sintered agglomerates of a plurality of abrasive grains and a binding material, the binding material being characterized by a melting temperature between 500 and 1400° C., and the sintered agglomerates having a loose packing density of ≦1.6 g/cc and three-dimensional shape; a bond material; and about 35-80 volume % total porosity, including at least 30 volume % interconnected porosity. Methods for making the sintered agglomerates and abrasive tools containing the sintered agglomerates are described.

This application is a continuation application of U.S. Ser. No.10/668,531 filed Sep. 23, 2003 U.S. Pat. No. 7,077,723, which is adivisional application Ser. No. 10/120,969 filed Apr. 11, 2002 U.S. Pat.No. 6,679,758.

BACKGROUND OF THE INVENTION

The invention relates to bonded abrasive articles or grinding tools madeporous by the use of certain agglomerated abrasive grains and to methodsfor making the agglomerated abrasive grains.

Grinding tools are manufactured in a variety of grades or structuresdetermined by the relative volume percentage of abrasive grain, bond andporosity within a composite abrasive grain matrix. In many grindingoperations, grinding tool porosity, particularly porosity of apermeable, or an interconnected nature, improves efficiency of thegrinding operation and quality of the work-piece being ground. Porosityinducers, such as bubble alumina and naphthalene, may be added toabrasive composite mixtures to permit pressure molding and handling of aporous uncured abrasive article and to yield an adequate volume percentporosity in the final tool.

Natural porosity arising from packing of the abrasive grains and bondparticles during pressure molding is insufficient to achieve a porositycharacter that is desirable for some grinding operations. Pore inducershave been added to achieve high porosity percentages, however, openchannels or interconnected porosity cannot be achieved with the poreinducers known in the art (e.g., hollow ceramic or glass spheres). Somepore induces must be burnt out of the abrasive matrix (e.g., walnutshells and naphthalene), giving rise to various manufacturingdifficulties. Further, the densities of pore inducers, bond materialsand abrasive grains vary significantly, often causing stratification ofthe abrasive mix during handling and molding, and, in turn, loss ofhomogeneity in the three-dimensional structure of the finished abrasivearticle. The volume percent of interconnected porosity, or fluidpermeability, has been found to be a more significant determinant ofgrinding performance of abrasive articles than mere volume percentporosity. U.S. Pat. No. 5,738,696 to Wu discloses a method for makingbonded abrasives utilizing elongated abrasive grain having an aspectratio of at least 5:1. The bonded abrasive aspect ratio of at least 5:1.The bonded abrasive wheels have a permeable structure containing 55-80%,by volume, of interconnected porosity. The interconnected porosityallows removal of grinding waste (swarf) and passage of cooling fluidwithin the wheel during grinding. The existence of interconnectedporosity is confirmed by measuring the permeability of the wheel to theflow of air under controlled conditions. The filamentary abrasive grainsare not agglomerated or otherwise coated with bond prior to assemblingthe wheel. U.S. Pat. No. 5,738,697 to Wu discloses high permeabilitygrinding wheels having a significant amount of interconnected porosity(40-80%, by volume). These wheels are made from a matrix of fibrousparticles having an aspect ratio of at least 5:1. The fibrous particlesmay be sintered sol gel alumina abrasive grain or ordinary, non-fibrousabrasive grains blended with various fibrous filler materials such asceramic fiber, polyester fiber and glass fiber and mats and agglomeratesconstructed with the fiber particles. The filamentary abrasive grainsare not agglomerated or otherwise coated with bond prior to assemblingthe wheel.

Abrasive grain has been agglomerated for various purposes, principalamong them to allow use of a smaller particle (grit) size to achieve thesame grinding efficiency as a larger abrasive grit size. In manyinstances abrasive grain has been agglomerated with bond materials toachieve a less porous structure and a denser grinding tool, having morestrongly bonded abrasive grains. Agglomerated abrasive grains have beenreported to improve grinding efficiency by mechanisms entirely unrelatedto the amount or character of the porosity of the abrasive article.

U.S. Pat. No. 2,194,472 to Jackson discloses coated abrasive tools madewith agglomerates of a plurality of relatively fine abrasive grain andany of the bonds normally used in coated or bonded abrasive tools.Organic bonds are used to adhere the agglomerates to the backing of thecoated abrasives. The agglomerates lend an open-coat face to coatedabrasives made with relatively fine grain. The coated abrasives madewith the agglomerates in place of individual abrasive grains arecharacterized as being relatively fast cutting, long-lived and suitablefor preparing a fine surface finish quality in the work-piece.

U.S. Pat. No. 2,216,728 to Benner discloses abrasive grain/bondaggregates made from any type of bond. The object of the aggregates isto achieve very dense wheel structures for retaining diamond or CBNgrain during grinding operations. If the aggregates are made with aporous structure, then it is for the purpose of allowing theinter-aggregate bond materials to flow into the pores of the aggregatesand fully densify the structure during firing. The aggregates allow theuse of abrasive grain fines otherwise lost in production.

U.S. Pat. No. 3,048,482 to Hurst discloses shaped abrasivemicro-segments of agglomerated abrasive grains and organic bondmaterials in the form of pyramids or other tapered shapes. The shapedabrasive micro-segments are adhered to a fibrous backing and used tomake coated abrasives and to line the surface of thin grinding wheels.The invention is characterized as yielding a longer cutting life,controlled flexibility of the tool, high strength and speed safety,resilient action and highly efficient cutting action relative to toolsmade without agglomerated abrasive grain micro-segments.

U.S. Pat. No. 3,982,359 to Elbel teaches the formation of resin bond andabrasive grain aggregates having a hardness greater than that of theresin bond used to bond the aggregates within an abrasive tool. Fastergrinding rates and longer tool life are achieved in rubber bonded wheelscontaining the aggregates.

U.S. Pat. No. 4,355,489 to Heyer discloses an abrasive article (wheel,disc, belt, sheet, block and the like) made of a matrix of undulatedfilaments bonded together at points of manual contact and abrasiveagglomerates, having a void volume of about 70-97%. The agglomerates maybe made with vitrified or resin bonds and any abrasive grain.

U.S. Pat. No. 4,364,746 to Bitzer discloses abrasive tools comprisingdifferent abrasive agglomerates having different strengths. Theagglomerates are made from abrasive grain and resin binders, and maycontain other materials, such as chopped fibers, for added strength orhardness.

U.S. Pat. No. 4,393,021 to Eisenberg, et al, discloses a method formaking abrasive agglomerates from abrasive grain and a resin binderutilizing a sieve web and rolling a paste of the grain and binderthrough the web to make worm-like extrusions. The extrusions arehardened by heating and then crushed to form agglomerates.

U.S. Pat. No. 4,799,939 to Bloecher teaches erodable agglomerates ofabrasive grain, hollow bodies and organic binder and the use of theseagglomerates in coated abrasives and bonded abrasives. Higher stockremoval, extended life and utility in wet grinding conditions areclaimed for abrasive articles comprising the agglomerates. Theagglomerates are preferably 150-3,000 microns in largest dimension. Tomake the agglomerates, the hollow bodies, grain, binder and water aremixed as a slurry, the slurry is solidified by heat or radiation toremove the water, and the solid mixture is crushed in a jaw or rollcrusher and screened.

U.S. Pat. No. 5,129,189 to Wetshcer discloses abrasive tools having aresin bond matrix containing conglomerates of abrasive grain and resinand filler material, such as cryolite.

U.S. Pat. No. 5,651,729 to Benguerel teaches a grinding wheel having acore and an abrasive rim made from a resin bond and crushed agglomeratesof diamond or CBN abrasive grain with a metal or ceramic bond. Thestated benefits of the wheels made with the agglomerates include highchip clearance spaces, high wear resistance, self-sharpeningcharacteristics, high mechanical resistance of the wheel and the abilityto directly bond the abrasive rim to the core of the wheel. In oneembodiment, used diamond or CBN bonded grinding rims are crushed to asize of 0.2 to 3 mm to form the agglomerates.

U.S. Pat. No. 4,311,489 to Kressner discloses agglomerates of fine (≦200micron) abrasive grain and cryolite, optionally with a silicate binder,and their use in making coated abrasive tools.

U.S. Pat. No. 4,541,842 to Rostoker discloses coated abrasives andabrasive wheels made with aggregates of abrasive grain and a foam madefrom a mixture of vitrified bond materials with other raw materials,such as carbon black or carbonates, suitable for foaming during firingof the aggregates. The aggregate “pellets” contain a larger percentageof bond than grain on a volume percentage basis. Pellets used to makeabrasive wheels are sintered at 900° C. (to a density of 70 lbs/cu. ft.;1.134 g/cc) and the vitrified bond used to make the wheel is fired at880° C. Wheels made with 16 volume % pellets performed in grinding withan efficiency similar to that of comparative wheels made with 46 volume% abrasive grain. The pellets contain open cells within the vitrifiedbond matrix, with the relative smaller abrasive grains clustered aroundthe perimeter of the open cells. A rotary kiln is mentioned for firingpre-agglomerated green aggregates to later foam and sinter to make thepellets.

U.S. Pat. No. 5,975,988 to Christianson discloses coated abrasivearticles include a backing and an organic bonded abrasive layer wherethe abrasive is present as shaped agglomerates in the shape of atruncated four-sided pyramid or cube. The agglomerates are made fromsuperabrasive grains bonded in an inorganic binder having a coefficientof thermal expansion which is the same or substantially the same as acoefficient of thermal expansion of the abrasive grain.

WO 00/51788 to Stoetzel, et al, discloses abrasive articles have abacking, an organic bond containing hard inorganic particles dispersedwithin it, and abrasive particle agglomerates bonded to the backing. Theabrasive particles in the agglomerates and the hard inorganic particlesin the organic bond are essential the same size. Agglomerates may berandomly or precisely shaped and they are made with an organic bond. Thehard inorganic particles may be any of a number of abrasive grainparticles.

U.S. Pat. No. 6,086,467 to Imai, et al, discloses grinding wheelscontain abrasive grain and grain clusters of filler grain having asmaller size than the abrasive grain. Vitrified bond may be used and thefiller grain may be chromium oxide. The size of the grain clusters is ⅓or more of the size of the abrasive grain. Benefits include controlledbond erosion and abrasive grain retention in low force grindingapplications utilizing superabrasive grain wherein the superabrasivegrain must be diluted to minimize grinding forces. Clusters of fillergrain may be formed with wax. No sintering of the clusters is disclosed.

WO 01/04227 A2 to Adefris, et al, discloses an abrasive articlecomprises a rigid backing and ceramic abrasive composites made ofabrasive particles in a porous ceramic matrix. The composites are heldto the backing with a metal coating, such an electroplated metal.

None of these prior art developments suggest the manufacture of abrasivearticles using porous agglomerated abrasive grain and bond particles tocontrol the percentage and character of porosity and to maintainporosity in the form of permeable, interconnected porosity in bondedabrasive articles. No suggestion is made to use a rotary calciner methodto manufacture a variety of abrasive grain agglomerates for use in theabrasive articles. The methods and tools of the invention yield newstructures from agglomerated mixtures of existing abrasive grain andbond combinations, and they are sophisticated in permitting thecontrolled design and manufacture of broad ranges of abrasive articlestructures having beneficial, bi-modal, interconnected porositycharacteristics. Such bi-modal, interconnected porosity enhancesabrasive tool performance, particularly in large contact area,precision-grinding operations, such as creepfeed surface grinding, innerdiameter grinding and toolroom grinding processes.

SUMMARY OF THE INVENTION

The invention is a bonded abrasive tool, having a structure permeable tofluid flow, the tool comprising:

-   a) about 5-75 volume % sintered agglomerates, comprising a plurality    of abrasive grains held with a binding material, the binding    material being characterized by a melting temperature between 500    and 1400° C., and the sintered agglomerates having a three    dimensional shape and an initial size distribution prior to    manufacture of the tool;-   b) a bond; and-   c) about 35-80 volume % total porosity, the porosity including at    least 30 volume % interconnected porosity;    wherein at least 50%, by weight, of the sintered agglomerates within    the bonded abrasive tool retain a plurality of abrasive grains held    in a three-dimensional shape after manufacture of the tool.

In another embodiment, the invention includes a vitrified bondedabrasive tool, having a structure permeable to fluid flow, the toolcomprising:

-   a) about 5-75 volume % sintered agglomerates of a plurality of    abrasive grain with a binding material, the binding material being    characterized by a viscosity A at the binding material melting    temperature;-   b) a vitrified bond characterized by a viscosity B at the binding    material melting temperature, viscosity B being at least 33% lower    than viscosity A; and-   c) about 35-80 volume % porosity, including at least 30 volume %    interconnected porosity.

The invention further includes a vitrified bonded abrasive tool, havinga structure permeable to fluid flow, the tool comprising:

-   a) about 5-60 volume % sintered agglomerates of a plurality of    abrasive grain with a binding material, the binding material being    characterized by a melting temperature A;-   b) a vitrified bond characterized by a melting temperature B,    melting temperature B being at least 150° C. lower than melting    temperature A; and-   c) about 35-80 volume % porosity, including at least 30 volume %    interconnected porosity.

In another aspect of the invention, the tool is a bonded abrasive tool,having a structure permeable to fluid flow, the tool comprising:

-   a) about 34-56 volume % abrasive grain;-   b) about 3-25 volume % bond; and-   c) about 35-80 volume % total porosity, including at least 30 volume    % interconnected porosity;    wherein the interconnected porosity has been created without the    addition of porosity inducing media and without the addition of    elongated shaped materials having a length to cross-sectional width    aspect ratio of at least 5:1.

The invention further includes processes for making the agglomerates andthe tools of the invention.

The invention includes a method of agglomerating abrasive grain,comprising the steps:

-   a) feeding the grain and a binding material, selected from the group    consisting essentially of vitrified bond materials, vitrified    materials, ceramic materials, inorganic binders, organic binders,    water, solvent and combinations thereof, into a rotary calcination    kiln at a controlled feed rate;-   b) rotating the kiln at a controlled speed;-   c) heating the mixture at a heating rate determined by the feed rate    and the speed of the kiln to temperatures from about 145 to 1,300°    C.,-   d) tumbling the grain and the binding material in the kiln until the    binding material adheres to the grain and a plurality of grains    adhere together to create a plurality of sintered agglomerates; and-   e) recovering the sintered agglomerates from the kiln, whereby the    sintered agglomerates have an initial three-dimensional shape, a    loose packing density of ≦1.6 g/cc and comprise a plurality of    abrasive grains.

The invention also includes sintered agglomerates of abrasive grain,made by a method comprising the steps:

-   a) feeding abrasive grain with a binding material into a rotary    calcination kiln at a controlled feed rate;-   b) rotating the kiln at a controlled speed;-   c) heating the mixture at a heating rate determined by the feed rate    and the speed of the kiln to temperatures from about 145 to 1,300°    C.,-   d) tumbling the grain and the binding material in the kiln until the    binding material adheres to the grain and a plurality of grains    adhere together to create a plurality of sintered agglomerates; and-   e) recovering the sintered agglomerates from the kiln, whereby the    sintered agglomerates have an initial three-dimensional shape, a    loose packing density of ≦1.6 g/cc and contain a plurality of    abrasive grains.

Using this process, an abrasive tool, comprising 5 to 75 volume %abrasive grain agglomerates, is made by a method comprising the steps:

-   a) feeding abrasive grain and a binding material, selected from the    group consisting essentially of vitrified bond materials, vitrified    materials, ceramic materials, inorganic binders, organic binders and    combinations thereof, into a rotary calcination kiln at a controlled    feed rate;-   b) rotating the kiln at a controlled speed;-   c) heating the mixture at a heating rate determined by the feed rate    and the speed of the kiln to temperatures from about 145 to 1,300°    C.,-   d) tumbling the mixture in the kiln until the binding material    adheres to the grain and a plurality of grains adhere together to    create a plurality of sintered agglomerates;-   e) recovering the sintered agglomerates from the kiln, the sintered    agglomerates consisting of a plurality of abrasive grains bonded    together by the binding material and having an initial    three-dimensional shape and a loose packing density of ≦1.6 g/cc;-   f molding the sintered agglomerates into a shaped composite body;    and-   g) thermally treating the shaped composite body to form the abrasive    tool.

Methods of grinding using the abrasive tools of the invention, inparticular, methods of surface grinding, also are disclosed.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a rotary kiln for carrying out theprocess for manufacturing the abrasive grain agglomerates of theinvention.

FIG. 2 is a photomicrograph of a cross-section of an abrasive wheel ofthe invention made with agglomerated grain (lighter areas of photos),and having intra-agglomerate porosity (smaller darker areas of photo)and inter-agglomerate, interconnected porosity (darker areas of photo).

FIG. 3 is a photomicrograph of a cross-section of a comparative abrasivewheel of the prior art, showing the absence of agglomerated grain andthe absence of large interconnected porosity in the structure of thewheel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Abrasive Agglomerates

Abrasive grain agglomerates of the invention are three-dimensionalstructures or granules, including sintered porous composites of abrasivegrain and binding material. The agglomerates have a loose packingdensity (LPD) of ≦1.6 g/cc, an average dimension of about 2 to 20 timesthe average abrasive grit size, and a porosity of about 30 to 88%, byvolume. The abrasive grain agglomerates preferably have a minimum crushstrength value of 0.2 MPa.

The abrasive grain may include one or more of the abrasive grains knownfor use in abrasive tools, such as the alumina grains, including fusedalumina, sintered and sol gel sintered alumina, sintered bauxite, andthe like, silicon carbide, alumina-zirconia, aluminoxynitride, ceria,boron suboxide, garnet, flint, diamond, including natural and syntheticdiamond, cubic boron nitride (CBN), and combinations thereof. Any sizeor shape of abrasive grain may be used. For example, the grain mayinclude elongated sintered sol gel alumina grains having a high aspectratio of the type disclosed in U.S. Pat. No. 5,129,919.

Grain sizes suitable for use herein range from regular abrasive grits(e.g., greater than 60 and up to 7,000 microns) to microabrasive grits(e.g., 0.5 to 60 microns), and mixtures of these sizes. For a givenabrasive grinding operation, it may be desirable to agglomerate anabrasive grain with a grit size smaller than an abrasive grain(non-agglomerated) grit size normally selected for this abrasivegrinding operation. For example, agglomerated 80 grit size abrasive maybe substituted for 54 grit abrasive, agglomerated 100 grit for 60 gritabrasive and agglomerated 120 grit for 80 grit abrasive.

The preferred sintered agglomerate size for typical abrasive grainsranges from about 200 to 3,000, more preferably 350 to 2,000, mostpreferably 425 to 1,000 micrometers in average diameter. Formicroabrasive grain, preferred sintered agglomerate size ranges from 5to 180, more preferably 20 to 150, most preferably 70 to 120 micrometersin average diameter.

The abrasive grain is present at about 10 to 65 volume %, morepreferably 35 to 55 volume %, and most preferably 48 to 52 volume % ofthe agglomerate.

Binding materials useful in making the agglomerates preferably includeceramic and vitrified materials, preferably of the sort used as bondsystems for vitrified bonded abrasive tools. These vitrified bondmaterials may be a pre-fired glass ground into a powder (a frit), or amixture of various raw materials such as clay, feldspar, lime, borax,and soda, or a combination of fritted and raw materials. Such materialsfuse and form a liquid glass phase at temperatures ranging from about500 to 1400° C. and wet the surface of the abrasive grain to create bondposts upon cooling, thus holding the abrasive grain within a compositestructure. Examples of suitable binding materials for use in theagglomerates are given in Table 2, below. Preferred binding materialsare characterized by a viscosity of about 345 to 55,300 poise at 1180°C., and by a melting temperature of about 800 to 1,300° C.

In a preferred embodiment, the binding material is a vitrified bondcomposition comprising a fired oxide composition of 71 wt % SiO₂ andB₂O₃, 14 wt % Al₂O₃, less than 0.5 wt % alkaline earth oxides and 13 wt% alkali oxides.

The binding material also may be a ceramic material, including, but notlimited to, silica, alkali, alkaline-earth, mixed alkali andalkaline-earth silicates, aluminum silicates, zirconium silicates,hydrated silicates, aluminates, oxides, nitrides, oxynitrides, carbides,oxycarbides and combinations and derivatives thereof. In general,ceramic materials differ from glassy or vitrified materials in that theceramic materials comprise crystalline structures. Some glassy phasesmay be present in combination with the crystalline structures,particularly in ceramic materials in an unrefined state. Ceramicmaterials in a raw state, such as clays, cements and minerals, may beused herein. Examples of specific ceramic materials suitable for useherein include, but are not limited to, silica, sodium silicates,mullite and other alumino silicates, zirconia-mullite, magnesiumaluminate, magnesium silicate, zirconium silicates, feldspar and otheralkali-alumino-silicates, spinels, calcium aluminate, magnesiumaluminate and other alkali aluminates, zirconia, zirconia stabilizedwith yttria, magnesia, calcia, cerium oxide, titania, or other rareearth additives, talc, iron oxide, aluminum oxide, bohemite, boronoxide, cerium oxide, alumina-oxynitride, boron nitride, silicon nitride,graphite and combinations of these ceramic materials.

The binding material is used in powdered form and may be added to aliquid vehicle to insure a uniform, homogeneous mixture of bindingmaterial with abrasive grain during manufacture of the agglomerates.

A dispersion of organic binders is preferably added to the powderedbinding material components as molding or processing aids. These bindersmay include dextrins, starch, animal protein glue, and other types ofglue; a liquid component, such as water, solvent, viscosity or pHmodifiers; and mixing aids. Use of organic binders improves agglomerateuniformity, particularly the uniformity of the binding materialdispersion on the grain, and the structural quality of the pre-fired orgreen agglomerates, as well as that of the fired abrasive toolcontaining the agglomerates. Because the binders burn off during firingof the agglomerates, they do not become part of the finished agglomeratenor of the finished abrasive tool.

An inorganic adhesion promoter may be added to the mixture to improveadhesion of the binding materials to the abrasive grain as needed toimprove the mix quality. The inorganic adhesion promoter may be usedwith or without an organic binder in preparing the agglomerates.

Although high temperature fusing binding materials are preferred in theagglomerates of the invention, the binding material also may compriseother inorganic binders, organic binders, organic bond materials, metalbond materials and combinations thereof. Binding materials used in theabrasive tool industry as bonds for organic bonded abrasives, coatedabrasives, metal bonded abrasives and the like are preferred.

The binding material is present at about 0.5 to 15 volume %, morepreferably 1 to 10 volume %, and most preferably 2 to 8 volume % of theagglomerate.

The preferred volume % porosity within the agglomerate is as high astechnically possible within the agglomerate mechanical strengthlimitations needed to manufacture an abrasive tool and to grind with it.Porosity may range from 30 to 88 volume %, preferably 40 to 80 volume %and most preferably, 50-75 volume %. A portion (e.g., up to about 75volume %) of the porosity within the agglomerates is preferably presentas interconnected porosity, or porosity permeable to the flow of fluids,including liquids (e.g., grinding coolant and swarf) and air.

The density of the agglomerates may be expressed in a number of ways.The bulk density of the agglomerates may be expressed as the LPD. Therelative density of the agglomerates may be expressed as a percentage ofinitial relative density, or as a ratio of the relative density of theagglomerates to the components used to make the agglomerates, takinginto account the volume of interconnected porosity in the agglomerates.

The initial average relative density, expressed as a percentage, may becalculated by dividing the LPD (ρ) by a theoretical density of theagglomerates (ρ₀), assuming zero porosity. The theoretical density maybe calculated according to the volumetric rule of mixtures method fromthe weight percentage and specific gravity of the binding material andof the abrasive grain contained in the agglomerates. For the sinteredagglomerates of the invention, a maximum percent relative density is 50volume %, with a maximum percent relative density of 30 volume % beingmore preferred.

The relative density may be measured by a fluid displacement volumetechnique so as to include interconnected porosity and exclude closedcell porosity. The relative density is the ratio of the volume of thesintered agglomerate measured by fluid displacement to the volume of thematerials used to make the sintered agglomerate. The volume of thematerials used to make the agglomerate is a measure of the apparentvolume based on the quantities and packing densities of the abrasivegrain and binder material used to make the agglomerates. For thesintered agglomerates of the invention, a maximum relative density ofthe sintered agglomerates preferably is 0.7, with a maximum relativedensity of 0.5 being more preferred.

Method of Manufacture of Abrasive Agglomerates

The agglomerates may be formed by a variety of techniques into numeroussizes and shapes. These techniques may be carried out before, during orafter firing the initial (“green”) stage mixture of grain and bindingmaterial. The step of heating the mixture to cause the binding materialto melt and flow, thus adhering the binding material to the grain andfixing the grain in an agglomerated form is referred to as firing,calcining or sintering. Any method known in the art for agglomeratingmixtures of particles may be used to prepare the abrasive agglomerates.

In a first embodiment of the process used herein to make agglomerates,the initial mixture of grain and binding material is agglomerated beforefiring the mixture so as to create a relatively weak mechanicalstructure referred to as a “green agglomerate” or “pre-firedagglomerate.”

To carry out the first embodiment, the abrasive grain and bindingmaterials may be agglomerated in the green state by a number ofdifferent techniques, e.g., in a pan pelletizer, and then fed into arotary calcination apparatus for sintering. The green agglomerates maybe placed onto a tray or rack and oven fired, without tumbling, in acontinuous or batch process.

The abrasive grain may be conveyed into a fluidized bed, then wettedwith a liquid containing the binding material to adhere the bindingmaterial to the grain, screened for agglomerate size, and then fired inan oven or calcination apparatus.

Pan pelletizing may be carried out by adding grain to a mixer bowl, andmetering a liquid component containing the binding material (e.g.,water, or organic binder and water) onto the grain, with mixing, toagglomerate them together. A liquid dispersion of the binding material,optionally with an organic binder, may be sprayed onto the grain, andthen the coated grain may be mixed to form agglomerates.

A low-pressure extrusion apparatus may be used to extrude a paste ofgrain and binding material into sizes and shapes which are dried to formagglomerates. A paste may be made of the binding materials and grainwith an organic binder solution and extruded into elongated particleswith the apparatus and method disclosed in U.S. Pat. No. 4,393,021.

In a dry granulation process, a sheet or block made of abrasive grainimbedded in dispersion or paste of the binding material may be dried andthen a roll compactor may be used to break the composite of grain andbinding material.

In another method of making green or precursor agglomerates, the mixtureof the binding material and the grain may be added to a molding deviceand the mixture molded to form precise shapes and sizes, for example, inthe manner disclosed in U.S. Pat. No. 6,217,413 B1.

In a second embodiment of the process useful herein for makingagglomerates, a simple mixture of the grain and binding material(optionally with an organic binder) is fed into a rotary calcinationapparatus of the type shown in FIG. 1. The mixture is tumbled at apredetermined rpm, along a predetermined incline with the application ofheat. Agglomerates are formed as the binding material mixture heats,melts, flows and adheres to the grain. The firing and agglomerationsteps are carried out simultaneously at controlled rates and volumes offeeding and heat application. The feed rate generally is set to yield aflow occupying roughly 8-12%, by volume, of the tube (i.e., the kilnportion) of the rotary calcination apparatus. The maximum temperatureexposure within the apparatus is selected to keep the viscosity of thebinding materials in a liquid state at a viscosity of at least about1,000 poise. This avoids excessive flow of the binding material onto thesurface of the tube and loss of binding material from the surface of theabrasive grain.

A rotary calcination apparatus of the type illustrated in FIG. 1 may beused to carry out the agglomeration process for agglomerating and firingthe agglomerates in a single process step. As shown in FIG. 1, a feedhopper (10) containing the feedstock (11) mixture of binding materialsand abrasive grain is fed into a means (12) for metering the mixtureinto a hollow heating tube (13). The tube (13) is positioned at anincline angle (14) of approximately 0.5-5.0 degrees such that thefeedstock (11) can be gravity fed through the hollow tube (13).Simultaneously, the hollow tube (13) is rotated in the direction of thearrow (a) at a controlled rate of speed to tumble the feedstock (11) andthe heated mix (18) as they pass along the length of the hollow tube.

A portion of the hollow tube (13) is heated. In one embodiment, theheating portion may comprise three heating zones (15, 16, 17) having alength dimension (d1) of 60 inches (152 mm) along the length (d2) of 120inches (305 mm) of the hollow tube (13). The heating zones permit theoperator to control the processing temperature and to vary it as neededto sinter the agglomerates. In other models of the apparatus, the hollowtube may only comprise one or two heating zones, or it may comprise morethan three heating zones. Although not illustrated in FIG. 1, theapparatus is equipped with a heating device and mechanical, electronicand temperature control and sensing devices operative for carrying outthe thermal process. As can be seen in the cross-sectional view of thehollow tube (13), the feedstock (11) is transformed to a heated mix (18)within the tube and it exits the tube and is collected as agglomerategranules (19). The wall of the hollow tube has an inner diameterdimension (d3) which may range from 5.5 to 30 inches (14-76 mm) and adiameter (d4) which may range from 6 to 36 inches (15-91 mm), dependingupon the model and the type of material used to construct the hollowtube (e.g., refractory metal alloy, refractory brick, silicon carbide,mullite).

The incline angle of the tube may range from 0.5 to 5.0 degrees and therotation of the tube may operate at 0.5 to 10 rpm. The feed rate for asmall scale rotary calciner may range from about 5 to 10 kg/hour, and anindustrial production scale feed rate may range from about 227 to 910kg/hour. The rotary calciner may be heated to a sintering temperature of800 to 1400° C., and the feed material may be heated at a rate of up to200° C./minute as the feedstock enters the heated zone. Cooling occursin the last portion of the tube as the feedstock moves from a heatedzone to an unheated zone. The product is cooled, e.g., with a watercooling system, to room temperature and collected.

Suitable rotary calcination machines may be obtained from HarperInternational, Buffalo, N.Y., or from Alstom Power, Inc., Applied TestSystems, Inc., and other equipment manufacturers. The apparatusoptionally may be fitted with electronic, in-process control anddetection devices, a cooling system, various designs of feed apparatusand other optional devices.

When agglomerating abrasive grain with lower temperature curing (e.g.,about from about 145 to about 500° C.) binding materials, an alternativeembodiment of this rotary kiln apparatus may be used. The alternativeembodiment, a rotary dryer, is equipped to supply heated air to thedischarge end of the tube to heat the abrasive grain mixture, cure thebinding material, bonding it to the grain, and thereby agglomerate theabrasive grain as it is collected from the apparatus. As used herein,the term “rotary calcination kiln” includes such rotary dryer devices.

In a third embodiment of the process useful herein for makingagglomerates, a mixture of the abrasive grain, binding materials and anorganic binder system is fed into an oven, without pre-agglomeration andheated. The mixture is heated to a temperature high enough to cause thebinding material to melt, flow and adhere to the grain, then cooled tomake a composite. The composite is crushed and screened to make thesintered agglomerates.

In the fourth embodiment, the agglomerates are not sintered beforemaking the abrasive tool, rather the “green” agglomerates are moldedwith bond material to form a tool body and the body is fired to form theabrasive tool. In a preferred method of carrying out this process, ahigh viscosity (when melted to form a liquid) vitrified binding materialis used to agglomerate grain in the green state. The green agglomeratesare oven-dried and mixed with a second, preferably lower viscosity,vitrified bond composition and molded into the form of a green abrasivetool. This green tool is fired at a temperature that is effective tofuse, but to avoid flow of, the high viscosity vitrified bindingmaterial. The firing temperature is selected to be sufficiently high tofuse the binding material composition into a glass; therebyagglomerating the grain, and to cause the bond composition to flow, bondthe agglomerates and form the tool. It is not essential to selectdifferent viscosity materials and materials with different fusing ormelting temperatures to carry out this process. Other combinations ofbinding materials and bond materials known in the art may be used inthis technique for making abrasive tools from green state agglomerates.

Abrasive Tools

The bonded abrasive tools of the invention include abrasive grindingwheels, segmented wheels, discs, hones, stones and other monolithic, orsegmented, shaped abrasive composites. The abrasive tools of theinvention comprise about 5 to 75 volume %, preferably 10 to 60 volume %,most preferably 20 to 52 volume % abrasive grain agglomerates.

In a preferred embodiment, vitrified bonded abrasive tools compriseabout 3 to 25 volume %, more preferably 4 to 20 volume %, and mostpreferably 5 to 19 volume % bond. Together with the abrasive grainagglomerates and the bond, these tools comprise about 35 to 80 volume %porosity, this porosity including at least 30 volume % of interconnectedporosity, preferably 55 to 80 volume % porosity, this porosity includingat least 50 volume % interconnected porosity. The vitrified bondedabrasive tools may comprise 35 to 52 volume % sintered agglomerates, 3to 13 volume % vitrified bond and 35 to 70 volume % porosity.

The amount of interconnected porosity is determined by measuring thefluid permeability of the tool according to the method of U.S. Pat. No.5,738,696. As used herein, Q/P=the fluid permeability of an abrasivetool, where Q means flow rate expressed as cc of air flow, and P meansdifferential pressure. The term Q/P represents the pressure differentialmeasured between the abrasive tool structure and the atmosphere at agiven flow rate of a fluid (e.g., air). This relative permeability Q/Pis proportional to the product of the pore volume and the square of thepore size. Larger pore sizes are preferred. Pore geometry and abrasivegrain size are other factors affecting Q/P, with larger grit sizeyielding higher relative permeability.

The abrasive tools of the invention are characterized by higher fluidpermeability values than comparable prior art tools. As used herein,“comparable prior art tools” are those tools made with the same abrasivegrain and bond materials at the same porosity and bond volumepercentages as those of the invention. In general, abrasive tools of theinvention have fluid permeability values of about 30 to 100% higher thanthe values of comparable prior art abrasive tools. The abrasive toolspreferably are characterized by fluid permeability values at least 10%higher, more preferably at least 30% higher, than those of comparableprior art tools.

Exact relative fluid permeability parameters for particular agglomeratesizes and shapes, bond types and porosity levels may be determined bythe practitioner by applying D'Arcy's Law to empirical data for a giventype of abrasive tool.

The porosity within the abrasive wheel arises from the open spacingprovided by the natural packing density of the tool components,particularly the abrasive agglomerates, and, optionally, by addingconventional pore inducing media. Suitable pore inducing media includes,but is not limited to, hollow glass spheres, ground walnut shells,hollow spheres or beads of plastic material or organic compounds, foamedglass particles, bubble mullite and bubble alumina, and combinationsthereof. The tools may be manufactured with open-cell porosity inducers,such as beads of naphthalene, or other organic granules, which burn outduring firing of the tool to leave void spaces within the tool matrix,or they may be manufactured with closed cell, hollow pore inducing media(e.g., hollow glass spheres). Preferred abrasive tools of the inventioneither do not contain added pore inducer media, or contain a minoramount of added pore inducer media effective to yield an abrasive toolwith a porosity content of which at least 30%, by volume isinterconnected porosity.

The bonded abrasive tools of the invention have a porous structure. Inthis structure, the average diameter of the sintered agglomerates is nogreater than an average dimension of the interconnected porosity whenthe interconnected porosity is measured at a point of a maximum opening.

The finished tools optionally contain added secondary abrasive grains,fillers, grinding aids and pore inducing media, and combinations ofthese materials. The total volume % abrasive grain in the tools(agglomerated and non-agglomerated grain) may range from about 34 toabout 56 volume %, more preferably from about 36 to about 54 volume %,and most preferably from about 36 to about 46 volume % of the tool. Thebonded abrasive tools preferably have a density of less than 2.2 g/cc.

When an abrasive grain is used in combination with the abrasiveagglomerates, the agglomerates preferably provide from about 5 to about100 volume % of the total abrasive grain of the tool and more preferablyfrom about 30 to about 70 volume % of the total abrasive in the tool.When such secondary abrasive grains are used, these abrasive grainspreferably provide from about 0.1 to about 95 volume % of the totalabrasive grain of the tool, and more preferably, from about 30 to about70 volume %. Suitable secondary abrasive grains include, but are notlimited to, various aluminum oxides, sol gel alumina, sintered bauxite,silicon carbide, alumina-zirconia, aluminoxynitride, ceria, boronsuboxide, cubic boron nitride, diamond, flint and garnet grains, andcombinations thereof.

The abrasive tools of the present invention preferably are bonded with avitreous bond. Any of the various bonds known in the art of makingabrasive tools may be selected for use herein. Examples of suitablebonds may be found in U.S. Pat. Nos. 4,543,107; 4,898,597; 5,203,886;5,401,284; 5,536,283; 5,095,665; 5,863,308; and 5,094,672, which arehereby incorporated by reference.

After firing, these vitreous bond compositions preferably include, butare not limited to a combination of the following oxides: SiO₂, Al₂O₃,Na₂O, Li₂O, and B₂O₃. Other oxides, such as K₂O, ZnO, ZrO₂, and alkalineearth oxides, such as CaO, MgO and BaO, may be present. Cobalt oxide(CoO) and other color sources may be included where bond color isdesirable. Other oxides, such as Fe₂O₃, TiO₂ and P₂O₅, and othercompounds existing as impurities in the raw materials may be included inthe bond. Frits may be used in addition to raw (or unfired) bondmaterials, or in lieu of raw bond materials. The raw materials for thebond may include clay, kaolin, alumina, lithium carbonate, boraxpentahydrate or boric acid, soda ash, flint and wollastonite, and suchother bond materials as are known in the art. The vitrified bond may bea glassy material or a ceramic material, with or without amorphousregions.

Organic binders are preferably added to powdered bond components,fritted or raw, as molding or processing aids. These binders may includedextrins, starch, animal protein glue and other types of glue, a liquidcomponent, such as water, viscosity or pH modifiers and mixing aids. Useof binders improves wheel uniformity and the structural quality of thepre-fired or green pressed wheel and the fired wheel. Because thebinders are burned out during firing, they do not become part of thefinished bond or abrasive tool.

An inorganic adhesion promoter may be added to the mixture to improveadhesion of glass bonds to the abrasive grain agglomerates as neededduring mixing and molding processes. The inorganic adhesion promoter maybe used with or without an organic binder in preparing the agglomerates.

For some of the agglomerates, the abrasive tool may be made withoutadded bond material, provided sufficient binder material is present inthe tool to yield appropriate mechanical strength properties in theabrasive tool during manufacturing of the tool and use of the tool ingrinding operations. For example, an abrasive tool may be constructedfrom at least 70 volume % agglomerates, having a binding materialcontent of at least 5 volume % of the agglomerate.

The density and hardness of the abrasive tools are determined by theselection of the agglomerates, type of bond and other tool components,the porosity content, together with the size and type of mold andselected pressing process.

Abrasive wheels may be molded and pressed by any means known in the art,including hot, warm and cold pressing techniques. Care must be taken inselecting a molding pressure for forming the green wheels to avoidcrushing an excessive amount of the abrasive grain agglomerates (e.g.,more than 50%, by weight, of the agglomerates) and to preserve thethree-dimensional structure of the agglomerates. The appropriate maximumapplied pressure for making the wheels of the invention depends upon theshape, size, thickness and bond component of the abrasive wheel, andupon the molding temperature. In common manufacturing processes, themaximum pressure may range from about 3,100 to 20,000 lbs/sq. in (218 to1,406 Kg/sq. cm). Molding and pressing are preferably carried out atabout 775 to 1,550 Kg/sq. cm, more preferably at 465 to 1,085 Kg/sq. cm.The agglomerates of the invention have sufficient mechanical strength towithstand the molding and pressing steps carried out in typicalcommercial manufacturing processes for making abrasive tools.

The abrasive wheels may be fired by methods known to those skilled inthe art. The firing conditions are primarily determined by the actualbond and abrasives used, and by the type of binding material containedin the abrasive grain agglomerate. Depending upon the chemicalcomposition of the selected bond, a vitrified bond may be fired at 600to 1250° C., preferably 850 to 1200° C., to provide the mechanicalproperties necessary for grinding metals, ceramics or other materials.The vitrified bonded body further may also be impregnated after firingin a conventional manner with a grinding aid, such as sulfur, or with avehicle, such as epoxy resin, to carry a grinding aid into the pores ofthe wheel.

Selection of a suitable vitrified bond will depend upon whichagglomeration process is in use and whether a melt or fuse temperatureor viscosity differential must be maintained between the bond and thebinding material of the agglomerate.

In making a vitrified bonded grinding wheel or other abrasive tool fromthe abrasive agglomerates, one of several general techniques may beselected. In the first one, a relatively higher firing temperature(e.g., fuses at above about 1,000° C.), vitrified binding material isapplied to agglomerate the grain. Then a second, lower firingtemperature (e.g., fuses at about 650 to 975° C.), powdered, vitrifiedbond composition is mixed with the grain agglomerates and molded intothe form of an abrasive tool. The green stage tool is fired at the lowerfiring temperature of the second bond material to create a finishedabrasive tool. In a preferred embodiment, the vitrified bond has a bondfiring temperature at least 150° C. lower than the binding materialmelting or fusing temperature.

In the second technique, viscosity differentials between the melted orfused glasses in their liquid state are exploited to use the same firingtemperature for making the agglomerate and firing the abrasive wheel. Ahigh viscosity vitrified binding material is used to agglomerate grainin a first firing step. Then the fired agglomerates are mixed with asecond, lower viscosity vitrified bond composition and molded into theform of a green abrasive tool. The molded tool may be fired at about thesame temperature as the temperature of the first firing step used tomake the agglomerates, because, when in a hot, liquid state, the bindingmaterial will not thin excessively and run off the grain. The originalthree-dimensional configuration of the agglomerate thus can bemaintained.

In a preferred embodiment of this technique, the viscosity of thevitrified bond at the binding material melting temperature is at least33% lower than the viscosity of the binding material at its meltingtemperature. Thus, when the viscosity of the binding material is about345 to 55,300 poise at 1180° C., the preferred vitrified bond materialis characterized by a viscosity of about 30 to 37,000 poise at 1180° C.

In the third technique, an intermediate firing temperature (e.g., about850-975° C.) binding material is used to agglomerate grain, butagglomeration is done at a temperature higher than the fuse or melttemperature of the binding material (e.g., 1000-1200° C.). Theagglomerates are mixed with the same binding material being utilized asthe vitrified bond composition and the mixture is molded into the formof a green abrasive tool. The green tool is fired at a lower temperature(e.g., about 850-975° C.) than temperature used to melt the bindingmaterial to agglomerate the grain. The lower temperature is effective tobond the agglomerates together. This process maintains thethree-dimensional structure of the agglomerates because the first layerof binding material does not flow at the firing temperature of theabrasive tool.

In a fourth technique, the same composition is used as the bindingmaterial and the bond for the wheel and the agglomeration and the wheelare carried out at the same temperature. It is theorized that becausethe binding material has been fused to form a glass adhered to theabrasive grain during agglomeration, the properties of the bindingmaterials have been altered. Thus, the fused binding material within thesintered agglomerates flows at a higher temperature than the unfusedbond material, and the agglomerates retain their shape as the wheel isbeing fired. In a preferred embodiment the composition used for thebinding material and the bond contain some raw materials and do notconsist of a fritted glass composition.

In a fifth technique for making vitrified abrasive tools, the tool ismade without added bond material. The agglomerates are packed into atool mold, pressed and fired at a temperature in the range of about 500to 1400° C. to form the tool. The binding materials used to make theagglomerates comprise a vitrified bond composition and the bindingmaterial is present in a sufficient amount in the agglomerate (e.g.,about 5 to 15 volume % of the agglomerate) to bond the agglomeratestogether in the finished vitrified abrasive tool.

The agglomerates may be bonded with all known types of bonds, such asorganic or resin bonds and metal bonds, known in the art ofmanufacturing bonded abrasive tools. The volume percent range foragglomerates suitable for use in vitrified abrasive tools is alsosatisfactory for metal and organic bonded tools. The organic and metalbonded tools usually comprise higher volume percentages of bond andlower volume percentages of porosity than vitrified bonded tools, andthe abrasive grain content may be higher. The organic and metal bondedtools may be mixed, molded and cured or sintered according to variousprocessing methods, and with various proportions of abrasive grain oragglomerate, bond and porosity components as are known in the art. Theagglomerates of the invention may be used in single layer metal bondedtools, as well as in multi-layer, three-dimensional structures,monolithic tools and segmented matrix abrasive tools as are known in theart.

Grinding Applications

The abrasive tools of the invention include abrasive wheels, discs,hones and stones and sticks and they are particularly effective ingrinding applications having large surface area contact between theabrasive tool and the workpiece. Such applications or grindingoperations include, but are not limited to, creepfeed and otherprecision surface grinding, porous toolroom grinding operations,internal diameter grinding operations and in fine surface grinding ofceramics and other brittle workpieces.

Fine grinding or polishing operations using micron or submicron sizedabrasive grain will benefit from use of tools made with the agglomeratesof the invention. Relative to conventional superfinishing or polishingtools and systems, the tools of the invention made with such fine gritabrasive agglomerates will erode at lower grinding forces with little orno surface damage to the workpiece during precision finishing operations(e.g., to yield mirror finishes on glass and ceramic components). Toollife remains satisfactory due to the agglomerated structures,particularly in single layer tools, but also in three-dimensional matrixand slurry tools.

In precision shaped profile grinding, the friablity of the agglomeratescontributes to fewer dressing cycles. Due to the interconnected porosityof the tools, coolant supply and debris removal are enhanced resultingin cooler grinding operations, less thermal damage to the workpiece andless grinding machine wear. Because smaller grit size abrasive grains inagglomerated form give the grinding efficiency of a larger grit sizegrain, but leave a smoother surface finish, the ground work part qualityoften improves significantly.

The following Examples are provided by way of illustration of theinvention, and not by way of limitation.

EXAMPLE 1

A series of agglomerated abrasive grain samples were prepared in arotary calcination apparatus (electric fired model # HOU-5D34-RT-28,1,200° C. maximum temperature, 30 KW input, equipped with a 72″ (183 cm)long, 5.5″ (14 cm) inner diameter refractory metal tube, manufactured byHarper International, Buffalo, N.Y.). The refractory metal tube wasreplaced with a silicon carbide tube of the same dimensions, and theapparatus was modified to operate at a maximum temperature of 1,550° C.The process of agglomeration was carried out under atmosphericconditions, at a hot zone temperature control set point of 1,180° C.,with an apparatus tube rotation rate of 9 rpm, a tube incline angle of2.5 to 3 degrees, and a material feedrate of 6-10 kg/hour. The apparatusused was substantially identical to the apparatus illustrated in FIG. 1.The yield of usable free-flowing granules (defined as −12 mesh to pan)was 60 to 90% of the total weight of the feedstock before calcination.

The agglomerate samples were made from a simple mixture of abrasivegrain, binding material and water mixtures described in Table 1-1. Thevitrified bond binding material compositions used to prepare the samplesare listed in Table 2. Samples were prepared from three types ofabrasive grains: fused alumina 38A, fused alumina 32A and sintered solgel alpha-alumina Norton SG grain, obtained from Saint-Gobain Ceramics &Plastics, Inc., Worcester, Mass., USA, in the grit sizes listed in Table1.

After agglomeration in the rotary calcination apparatus, theagglomerated abrasive grain samples were screened and tested for loosepacking density (LPD), size distribution and agglomerate strength. Theseresults are shown in Table 1.

TABLE 1-1 Agglomerated Granule Characteristics Sample Binding No.material pressure grain Weight Weight Volume LPD Average Average at 50%liquid lbs % (on % of g/cc size size Average % crushed binding (Kg)grain binding −12/ distribution distribution relative fraction materialof mix basis) material^(a) pan microns mesh size density MPa 1 2.0 3.181.46 334 −40/+50 41.0 0.6 ± 0.1 60 grit 30.00 (13.6) 38A 0.60 water(0.3) A binding 0.64 material (0.3) 2 6.0 8.94 1.21 318 −45/+50 37.0 0.5± 0.1 90 grit 30.00 (13.6) 38A 0.90 water (0.4) E binding 1.99 material(0.9) 3 10.0 13.92 0.83 782 −20/+25 22.3 2.6 ± 0.2 120 grit 30.00 (13.6)38A 1.20 water (0.5) C binding 3.41 material (1.5) 4 6.0 8.94 1.13 259−50/+60 31.3 0.3 ± 0.1 120 grit 30.00 (13.6) 32A 0.90 water (0.4) Abinding 1.91 material (0.9) 5 10.0 14.04 1.33 603 −25/+30 37.0 3.7 ± 0.260 grit 30.00 (13.6) 32A 1.20 water (0.5) E binding 3.31 material (1.5)6 2.0 3.13 1.03 423 −40/+45 28.4 0.7 ± 0.1 90 grit 30.00 (13.6) 32A 0.60water (0.3) C binding 0.68 material (0.3) 7 10.0 14.05 1.20 355 −45/+5036.7 0.5 ± 0.1 90 grit 30.00 (13.6) SG 1.20 water (0.5) A binding 3.18material (1.4) 8 2.0 3.15 1.38 120 −120/+140 39.1 — 120 grit 30.00(13.6) SG 0.60 water (0.3) E binding 0.66 material (0.3) 9 6.0 8.87 1.03973 −18/+20 27.6 — 60 grit 30.00 (13.6) SG 0.90 water (0.4) C binding2.05 material (0.9) ^(a)The volume % binding material is a percentage ofthe solid material within the granule (i.e., binding material and grain)after firing, and does not include the volume % porosity.

The volume % binding material of the fired agglomerates was calculatedusing the average LOI (loss on ignition) of the binding material rawmaterials.

The sintered agglomerates were sized with U.S. standard testing sievesmounted on a vibrating screening apparatus (Ro-Tap; Model RX-29; W.S.Tyler Inc. Mentor, Ohio). Screen mesh sizes ranged from 18 to 140, asappropriate for different samples. The loose packed density of thesintered agglomerates (LPD) was measured by the American NationalStandard procedure for Bulk Density of Abrasive Grains.

The initial average relative density, expressed as a percentage, wascalculated by dividing the LPD (ρ) by a theoretical density of theagglomerates (ρ₀), assuming zero porosity. The theoretical density wascalculated according to the volumetric rule of mixtures method from theweight percentage and specific gravity of the binding material and ofthe abrasive grain contained in the agglomerates.

The strength of the agglomerates was measured by a compaction test. Thecompaction tests were performed using one inch (2.54 cm) in diameterlubricated steel die on an Instron® universal testing machine (model MTS1125, 20,000 lbs (9072 Kg)) with a 5 gram sample of agglomerate. Theagglomerate sample was poured into the die and slightly leveled bytapping the outside of the die. A top punch was inserted and a crossheadlowered until a force (“initial position”) was observed on the recorder.Pressure at a constant rate of increase (2 mm/min) was applied to thesample up to a maximum of 180 MPa of pressure. The volume of theagglomerate sample (the compacted LPD of the sample), observed as adisplacement of the crosshead (the strain), was recorded as the relativedensity as a function of the log of the applied pressure. The residualmaterial was then screened to determine the percent crush fraction.Different pressures were measured to establish a graph of therelationship between the log of the applied pressure and the percentcrush fraction. Results are reported in Table 1 as the log of thepressure at the point where the crush fraction equates to 50 weightpercent of the agglomerate sample. The crush fraction is the ratio ofthe weight of crushed particles passing through the smaller screen tothe weight of the initial weight of the sample.

These agglomerates had LPD, size distribution, and molding strength andgranule size retention characteristics suitable for use in thecommercial manufacture of abrasive grinding wheels. The finished,sintered agglomerates had three-dimensional shapes varying amongtriangular, spherical, cubic, rectangular and other geometric shapes.Agglomerates consisted of a plurality of individual abrasive grits(e.g., 2 to 20 grits) bonded together by glass binding material at gritto grit contact points.

Agglomerate granule size increased with an increase in amount of bindingmaterial in the agglomerate granule over the range from 3 to 20 weight %of the binding material.

Adequate compaction strength was observed for all samples 1-9,indicating that the glass binding material had matured and flowed tocreate an effective bond among the abrasive grains within theagglomerate. Agglomerates made with 10 weight % binding material hadsignificantly higher compaction strength than those made with 2 or 6weight % binding material.

Lower LPD values were an indicator of a higher degree of agglomeration.The LPD of the agglomerates decreased with increasing weight % bindingmaterial and with decreasing abrasive grit size. Relatively largedifferences between 2 and 6 weight % binding material, compared withrelatively small differences between 6 and 10 weight % binding materialindicate a weight % binding material of less than 2 weight % may beinadequate for formation of agglomerates. At the higher weightpercentages, above about 6 weight %, the addition of more bindingmaterial may not be beneficial in making significantly larger orstronger agglomerates.

As suggested by agglomerate granule size results, binding material Csamples, having the lowest molten glass viscosity at the agglomeratingtemperature, had the lowest LPD of the three binding materials. Theabrasive type did not have a significant effect upon the LPD.

TABLE 2 Binding Material used in the Agglomerates A Binding material D EF Fired wt % B Binding C Binding Binding Binding Binding Composition(A-1 binding material material material material material Elements^(b)material)^(a) wt % wt % wt % wt % wt % glass formers 69 69 71 73 64 68(SiO₂ + B₂O₃) (72) Al₂O₃ 15 10 14 10 18 16 (11) alkaline earth 5-6 <0.5<0.5 1-2 6-7 5-6 RO (CaO, (7-8) MgO) Alkali R₂O  9-10 20 13 15 11 10(Na₂O, K₂O, (10) Li₂O) Spec. Gravity 2.40 2.38 2.42 2.45 2.40 2.40 g/ccEstimated 25,590 30 345 850 55,300 7,800 Viscosity (Poise) at 1180° C.^(a)The A-1 binding material variation set forth in parentheses was usedfor the samples of Example 2. ^(b)Impurities (e.g., Fe₂O₃ and TiO₂) arepresent at about 0.1-2%.

EXAMPLE 2

Additional samples of agglomerates were made utilizing various otherprocessing embodiments and feedstock materials.

A series of agglomerates (sample nos. 10-13) were formed at differentsintering temperatures, ranging from 1100 to 1250° C., utilizing arotary calcination apparatus (model #HOU-6D60-RTA-28, equipped with a120 inch (305 cm) long, 5.75 inch (15.6 cm) inner diameter, ⅜ inch (0.95cm) thick, mullite tube, having a 60 inch (152 cm) heated length withthree temperature control zones. The apparatus was manufactured byHarper International, Buffalo, N.Y.). A Brabender feeder unit withadjustable control volumetric feed-rate was used to meter the abrasivegrain and binding material mixture into the heating tube of the rotarycalcination apparatus. The process of agglomeration was carried underatmospheric conditions, with an apparatus tube rotation rate of 4 rpm, atube incline angle of 2.5 degrees, and a feed rate of 8 kg/hour. Theapparatus used was substantially identical to the apparatus illustratedin FIG. 1. Temperature selections and other variables utilized to makethese agglomerates are set forth in Table 2-1.

All samples contained a mixture, on a weight % basis, of 89.86% abrasivegrain (60 grit 38A alumina grain obtained from Saint-Gobain Ceramics &Plastics, Inc.), 10.16% binder mixture (6.3 wt % AR30 liquid proteinbinder, 1.0% Carbowax® 3350 PEG and 2.86% of binding material A). Thismixture yielded 4.77 volume % binding material and 95.23 volume % grainin the sintered agglomerate granule. The calculated theoretical densityof the agglomerate granules (assuming no porosity) was 3.852 g/cc.

Prior to placing the mixture into the feeder unit, green stageagglomerates were formed by simulated extrusion. To prepare extrudedagglomerates, the liquid protein binder was heated to dissolve theCarbowax® 3350 PEG. Then the binding material was added slowly whilestirring the mixture. Abrasive grains were added to a high shear mixer(44 inch (112 cm) diameter) and the prepared binding material-bindermixture was slowly added to the grain in the mixer. The combination wasmixed for 3 minutes. The mixed combination was wet-screened through a 12mesh box screen (US standard sieve size) onto trays in a layer at amaximum depth of one inch (2.5 cm) to form wet, green (unfired),extruded agglomerates. The layer of extruded agglomerates was oven driedat 90° C. for 24 hours. After drying, the agglomerates were screenedagain using a 12 to 16 mesh (U.S. standard sieve size) box screen.

It was observed during rotary calcination that the agglomerates made inthe green state appeared to break apart when heated, and, then,re-formed as they tumbled out of the exit end of the heated portion ofthe rotary calciner tube. The larger size of the agglomerated granulesmade in the green state, relative to that of the agglomerated granulesafter firing, was readily apparent upon visual inspection of thesamples.

After firing, the agglomerated particle sizes were observed to besufficiently uniform for commercial purposes, with a size distributionover a range of about 500-1200 microns. The size distributionmeasurements are set forth in Table 2-2, below. Yield, size, crushstrength and LPD were acceptable for commercial use in making grindingwheels.

TABLE 2-1 pressure % LPD at 50% Sintering Yield Ave. g/cc crushed %yield Ave. agglom LPD g/cc Sample Temp.^(a) −12 size −12 fraction−16/+35 size −16/+35 No. ° C. mesh μm mesh MPa mesh μm mesh (10) 1100n/a^(b) n/a n/a n/a n/a 536 n/a (11) 1150 97.10 650 1.20 13 ± 1  76.20632 0.95 (12) 1200 96.20 750 1.20 9 ± 1 87.00 682 1.04 (13) 1250 96.60675 1.25 8 ± 1 85.20 641 1.04 ^(a)Temperature of rotary calcinercontroller set point (for all 3 zones). ^(b)“n/a” indicates nomeasurement was made.

TABLE 2-2 Particle size distribution for fired agglomerates Sieve #ASTM-E Sample Sieve # Weight % on Screen No. ISO 565 μm 10 11 12 13 −35−500 41.05 17.49 11.57 14.31 35 500 22.69 17.86 14.56 17.69 30 600 18.3024.34 21.27 26.01 25 725 12.57 21.53 24.89 23.06 20 850 3.43 13.25 16.1712.43 18 1000 1.80 4.58 10.09 5.97 16 1180 0.16 0.95 1.44 0.54

EXAMPLE 3

Agglomerates (sample nos. 14-23) were prepared as described in Example2, except the temperature was maintained constant at 1000° C., and amodel #KOU-8D48-RTA-20 rotary calciner apparatus, equipped with a 108inch (274 cm) long, 8 inch (20 cm) inner diameter, fused silica tube,having a 48 inch (122 cm) heated length with three temperature controlzones, was used. The apparatus was manufactured by Harper International,Buffalo, N.Y. Various methods were examined for preparation of thepre-fired mixture of grain and binding material. The process ofagglomeration was carried under atmospheric conditions, with anapparatus tube rotation rate of 3 to 4 rpm, a tube incline angle of 2.5degrees, and a feed rate of 8 to 10 kg/hour. The apparatus used wassubstantially identical to the apparatus illustrated in FIG. 1.

All samples contained 30 lbs (13.6 Kg) abrasive grain (the same grainused in Example 2, except that sample 16 contained 25 lbs (11.3 Kg) of70 grit Norton SG® sol gel alumina grain, obtained from Saint-GobainCeramics and Plastics, Inc.) and 0.9 lbs (0.41 Kg) binding material A(yielding 4.89 volume % binding material in the sintered agglomerate).The binding material was dispersed in different binder systems prior toaddition to the grain. The binder system of Example 2 (“Binder 2”) wasused for some samples and other samples were made using AR30 liquidprotein binder (“Binder 3”) in the weight percentages listed below inTable 3. Sample 20 was used to prepare agglomerates in the green,unfired state by the simulated extrusion method of Example 2.

The variables tested and the test results of the tests are summarizedbelow in Table 3.

TABLE 3 Green stage binder treatments wt % binder % Yield- (as % of 12Sample Mix grain mesh LPD No. Treatment wt) screen g/cc 14 Binder 3 2.0100 1.45 15 Binder 3 1.0 100 1.48 16 Binder 3; 4.0 92 1.38 SG grain 17Binder 3 4.0 98 1.44 18 Binder 2 6.3 90 1.35 19 Binder 3 8.0 93 1.30 20Binder 2; 6.3 100 1.37 simulated extrusion 21 Binder 3 3.0 100 1.40 22Binder 3 6.0 94 1.44 23 Binder 2 4.0 97 1.54

These results confirm that green stage agglomeration is not needed toform an acceptable quality and yield of sintered agglomerated granules(compare samples 18 and 20). As the wt % of Binder 3 used in the initialmix increased from 1 to 8%, the LPD showed a trend towards a moderatedecrease, indicating that the use of a binder has a beneficial, but notessential, effect upon the agglomeration process. Thus, ratherunexpectedly, it did not appear necessary to pre-form a desiredagglomerate granule shape or size prior to sintering it in a rotarycalciner. The same LPD was achieved merely by feeding a wet mixture ofthe agglomerate components into the rotary calciner and tumbling themixture as it passes through the heated portion of the apparatus.

EXAMPLE 4

Agglomerates (sample nos. 24-29) were prepared as described in Example2, except the temperature was maintained constant at 1200° C. andvarious methods were examined for preparation of the pre-fired mixtureof grain and binding material. All samples (except samples 28-29)contained a mixture of 300 lbs (136.4 Kg) abrasive grain (same grain asExample 2: 60 grit 38A alumina) and 9.0 lbs (4.1 Kg) of binding materialA (yielding 4.89 volume % binding material in the sintered agglomerate).

Sample 28 (same composition as Example 2) contained 44.9 lbs (20.4 Kg)of grain and 1.43 lbs (0.6Kg) of binding material A. The bindingmaterial was combined with the liquid binder mixture (37.8 wt % (3.1lbs) of AR30 binder in water) and 4.98 lbs of this combination was addedto the grain. The viscosity of the liquid combination was 784 CP at 22°C. (Brookfield LVF Viscometer).

Sample 29 (same composition as Example 2) contained 28.6 lbs (13 Kg) ofgrain and 0.92 lbs (0.4 Kg) of binding material A (yielding 4.89 volume% binding material in the sintered agglomerate). The binding materialwas combined with the liquid binder mixture (54.7 wt % (0.48 lbs)Duramax® resin B1052 and 30.1 wt % (1.456 lbs) Duramax resin B1051 resinin water) and this combination was added to the abrasive grain. TheDuramax resins were obtained from Rohm and Haas, Philadelphia, Pa.

The process of agglomeration was carried under atmospheric conditions,with an apparatus tube rotation rate of 4 rpm, a tube incline angle of2.5 degrees, and a feed rate of 8 to 12 kg/hour. The apparatus used wassubstantially identical to the apparatus illustrated in FIG. 1.

Sample 28 was pre-agglomerated, before calcination, in a fluidized bedapparatus made by Niro, Inc., Columbia, Md. (model MP-2/3Multi-Processor™, equipped with a MP-1 size cone (3 feet (0.9 meter) indiameter at its widest width). The following process variables wereselected for the fluidized bed process sample runs:

inlet air temperature 64-70° C.

inlet air flow 100-300 cubic meters/hour

granulation liquid flow rate 440 g/min

bed depth (initial charge 3-4 kg) about 10 cm

air pressure 1 bar

two fluid external mix nozzle 800 micron orifice

The abrasive grain was loaded into the bottom apparatus and air wasdirected through the fluidized bed plate diffuser up and into the grain.At the same time, the liquid mixture of binding material and binder waspumped to the external mix nozzle and then sprayed from the nozzlesthrough the plate diffuser and into the grain, thereby coatingindividual abrasive grits. Green stage agglomerates were formed duringthe drying of the binding material and binder mixture.

Sample 29 was pre-agglomerated, before calcination, in a low pressureextrusion process using a Benchtop Granulator™ made by LCI Corporation,Charlotte, N.C. (equipped with a perforated basket having 0.5 mmdiameter holes). The mixture of grain, binding material and binder wasmanually fed into the perforated basket (the extruder screen), forcedthrough the screen by rotating blades and collected in a receiving pan.The extruded pre-agglomerates were oven-dried at 90° C. for 24 hours andused as feed stock for the rotary calcination process.

The variables tested and the results of the tests are summarized belowand in Tables 4-1 and 4-2. These tests confirm the results set forth inExample 3 are also observed at a higher firing temperature (1200 versus1000° C.). These tests also illustrate that low-pressure extrusion andfluid bed pre-agglomeration may be used to make agglomerated granules,but an agglomeration step before rotary calcination is not necessary tomake the agglomerates of the invention.

TABLE 4-1 Agglomerate characteristics wt % binder % Yield on grain −12Average Sample Mix wt % mesh size LPD No. Treatment basis screen μm g/cc24 Binder 3 1.0 71.25 576 1.30 25 Binder 3 4.0 95.01 575 1.30 26 Binder3 8.0 82.63 568 1.32 27 Binder 2 7.2 95.51 595 1.35 28 Binder 3 7.290.39 n/a n/a 29 Duramax 7.2 76.17 600 1.27 resin

TABLE 4-2 Particle size distribution for agglomerates Sieve # ASTM-ESample Sieve # Weight % on Screen No. ISO 565 μm 24 25 26 27 28 29 −40−425 17.16 11.80 11.50 11.50 n/a 11.10 40 425 11.90 13.50 14.00 12.50n/a 12.20 35 500 17.30 20.70 22.70 19.60 n/a 18.90 30 600 20.10 25.2026.30 23.80 n/a 23.70 25 725 17.60 19.00 17.20 18.40 n/a 19.20 20 85010.80 8.10 6.40 9.30 n/a 10.30 18 1000 3.90 1.70 1.60 3.20 n/a 3.60 161180 0.80 0.10 0.30 1.60 n/a 1.10

EXAMPLE 5

Additional agglomerates (sample nos. 30-37) were prepared as describedin Example 3, except sintering was done at 1180° C., different types ofabrasive grains were tested, and 30 lbs (13.6 Kg) of abrasive grain wasmixed with 1.91 lbs (0.9 Kg) of binding material A (to yield 8.94 volume% binding material in the sintered agglomerate granules). Binder 3 ofExample 3 was compared with water as a binder for green stageagglomeration. Samples 30-34 used 0.9 lbs (0.4 Kg) of water as a binder.Samples 35-37 used 0.72 lbs (0.3 Kg) of Binder 3. The variables testedare summarized below in Table 5.

The process of agglomeration was carried under atmospheric conditions,with an apparatus tube rotation rate of 8.5-9.5 rpm, a tube inclineangle of 2.5 degrees, and a feed rate of 5-8 kg/hour. The apparatus usedwas substantially identical to the appartaus illustrated in FIG. 1.

After agglomeration, the agglomerated abrasive grain samples werescreened and tested for loose packing density (LPD), size distributionand agglomerate strength. These results are shown in Table 5.

TABLE 5 wt % pressure at binder 50% on grain Average crushed SampleAbrasive wt % size LPD fraction No. grain Binder basis μm g/cc MPa 30 60grit water 3.0 479 1.39 1.2 ± 0.1 57A alumina 31 60 grit water 3.0 5741.27 2.5 ± 0.1 55A alumina 32 80 grit water 3.0 344 1.18 0.4 ± 0.1 XGalumina 33 70 grit water 3.0 852 1.54  17 ± 1.0 Targa ® sol gel alumina34 70/30 wt % water 3.0 464 1.31 1.1 ± 0.1 60 grit 38A/60 grit Norton SGalumina 35 60 grit 38A Binder 3 2.4 n/a n/a n/a alumina 36 60 gritBinder 3 2.4 n/a n/a n/a Norton SG ® alumina 37 60/25/15 wt Binder 3 2.4n/a n/a n/a % 60 grit 38 A/120 grit Norton SG/ 320 grit 57A

These results again demonstrate the utility of water as a temporarybinder for the agglomerates in the rotary calcination process. Further,mixtures of grain types, grain sizes, or both, may be agglomerated bythe process of the invention and these agglomerates can be coated at atemperature of 1180° C. in the rotary calciner. A significant increasein crush strength was observed when a high aspect ratio (i.e., ≧4:1),elongated abrasive grain was used in the agglomerates (sample 33).

EXAMPLE 6

Another series of agglomerates (sample nos. 38-45) was prepared asdescribed in Example 3, except different sintering temperatures wereused, and different types of abrasive grain grit sizes blends, differentbinding materials were tested. In some of the feedstock mixtures, walnutshell was used as an organic pore inducer filler material (walnut shellwas obtained from Composition Materials Co., Inc., Fairfield, Conn., inUS Sieve size 40/60). The variables tested are summarized below in Table6. All samples contained a mixture of 30 lbs (13.6 Kg) abrasive grainand 2.5 wt % Binder 3, on grain weight basis, with various amounts ofbinding materials as shown in Table 6.

The process of agglomeration was carried under atmospheric conditions,with an apparatus tube rotation rate of 8.5-9.5 rpm, a tube inclineangle of 2.5 degrees, and a feed rate of 5-8 kg/hour. The apparatus usedwas substantially identical to the apparatus illustrated in FIG. 1.

After agglomeration, the agglomerated abrasive grain samples werescreened and tested for loose packing density (LPD), average size andagglomerate crush strength (see Table 6). The properties of allagglomerates were acceptable for use in manufacturing abrasive grindingwheels. These data appear to indicate the use of organic pore inducers,i.e., walnut shells, had no significant impact on agglomeratecharacteristics.

TABLE 6 Abrasive grain pressure at wt % Vol % Vol % 50% mixture FiredFired crushed Sample grit size Binding Binding Pore LPD fraction No.grain type material material^(a) Inducer g/cc MPa 38 90/10 wt % F 5.18 01.14 11.5 ± 0.5 60 grit 38A alumina/ 70 grit Targa ® sol gel alumina 3990/10 wt % C 7.88 2 1.00 11.5 ± 0.5 60 grit 38A alumina/ 70 grit Targa ®sol gel alumina 40 90/10 wt % F 5.18 2 1.02 10.5 ± 0.5 80 grit 38Aalumina/ 70 grit Targa ® sol gel alumina 41 90/10 wt % C 7.88 0 0.92 n/a80 grit 38A alumina/ 70 grit Targa ® sol gel alumina 42 50/50 wt % F5.18 2 1.16 11.5 ± 0.5 60 grit 38A alumina/ 60 grit 32A alumina 43 50/50wt % C 7.88 0 1.06 n/a 60 grit 38A alumina/ 60 grit 32A alumina 44 50/50vol % F 5.18 0 1.08  8.5 ± 0.5 80 grit 38A alumina/ 60 grit 32A alumina45 50/50 vol % C 7.88 2 1.07 11.5 ± 0.5 80 grit 38A alumina/ 60 grit 32Aalumina ^(a)Volume % is on the basis of total solids (grain, bindingmaterial and pore inducer) and does not include the porosity of theagglomerate.

EXAMPLE 7

Agglomerate samples 10-13 and 24-27 prepared according to Examples 2 and4, respectively, were used to make grinding wheels (finished size:20×1×8 inch) (50.8×2.54×20.3 cm). These wheels were tested in acreepfeed grinding operation against comparative wheels made withoutagglomerates, but containing pore inducer filler material.

To make the abrasive wheels, the agglomerates were added to a mixeralong with a liquid binder and a powdered vitrified bond compositioncorresponding to Binding material C from Table 1-2. The wheels were thenmolded, dried, fired to a maximum temperature of 900° C., graded,finished, balanced and inspected according to commercial grinding wheelmanufacturing techniques known in the art.

The composition of the wheels (including volume % abrasive, bond andporosity in the fired wheels), density, and modulus properties of thewheels are described in Table 7-1. Wheels were formulated to a modulusof elasticity corresponding to a standard wheel hardness grade betweenthe D and E grades on the Norton Company hardness grade scale.Preliminary tests had established the wheels formulated fromagglomerated grain with a volume % structure (i.e., volume % grain, bondand pores, to a total of 100%) identical to that of a comparative wheelmade without agglomerated grain were in fact considerably lower indensity, had a lower elastic modulus, and were softer than thecomparative wheel. Thus, density and elastic modulus, more so thancalculated volume % structure, were selected as the critical wheelhardness indicators for wheels made with agglomerated grain and testedin these grinding studies.

TABLE 7-1 Abrasive Wheel Characteristics Relative Fired Mod. of WheelComposition Air Density Elasticity Volume % Permeability^(b) g/cc d/cm²×10¹⁰ Wheel (agglomerate samples Ex. 2, 5) Agglom.^(a) Bond^(d) Porosity(10) 37.50 5.70 56.80 81.8 1.62 10.7 (11) 37.50 5.70 56.80 84.1 1.6110.6 (12) 37.50 5.70 56.80 87.8 1.60 11.1 (12) 37.50 5.70 56.80 89.51.60 10.2 (13) 37.50 5.70 56.80 79.2 1.61 11.4 (27) 37.50 8.40 54.1090.3 1.66 13.9 (26) 37.50 8.40 54.10 90.6 1.65 14.8 (26) 37.50 8.4054.10 80.1 1.65 15.4 (25) 37.50 8.40 54.10 n/a 1.66 15.6 (24) 37.50 8.4054.10 n/a 1.69 17.6 Comparative samples^(c) non- agglomerated Grain BondPorosity grain vol % Vol % vol % 38A60-D25VCF2 37.50^(a) 4.70 57.80 75.81.60 9.20 38A60-D25VCF2 37.50^(a) 4.70 57.80 75.8 1.59 9.6038A60-E25VCF2 37.50^(a) 5.70 56.80 59.6 1.67 19.80 38A60-D28VCF236.00^(a) 4.70 59.30 n/a 1.64 15.50 ^(a)At 37.50 vol. % abrasive grain,the comparative wheels contained a greater volume % abrasive grain(i.e., 1-3 volume % more) than the experimental wheels made with 37.50vol. % agglomerated grain, binding material and intra-agglomerateporosity. ^(b)Fluid (air) permeability was measured by the test methodsdisclosed in U.S. Pat. Nos. 5,738,696 and 5,738,697, assigned to NortonCompany. Relative air permeability values are expressed incc/second/inch of water units. ^(c)Comparative wheel samples werecommercial products obtained from Saint-Gobain Abrasives, Inc.,Worcester, MA, and marked with the wheel designations indicated for eachin Table 7-1. ^(d)Values for volume % bond of the experimental wheels donot include the volume % glass binding material used on the grains tomake the agglomerates. Volume % bond represents only the materials addedto make the grinding wheels.

The wheels were tested in a creepfeed grinding operation againstcomparative commercial wheels recommended for use in creepfeed grindingoperations (the comparative wheels are described in Tables 7-1 and 7-2).The comparative wheels had the same size dimensions, comparable hardnessgrades and were otherwise suitable comparative wheels to theexperimental wheels in a creepfeed grinding study, but they were madewithout agglomerates.

Grinding Conditions:

-   Machine: Hauni-Blohm Profimat 410-   Mode: Slot creepfeed grind-   Depth of Cut: 0.125 inch (0.318 cm)-   Wheel speed: 5500 surface feet per minute (28 m/sec)-   Table speed: Varied in increments of 2.5 in/min (6.4 cm/min.) from    5-17.5 inches/minute (12.7-44.4 cm/minute) or until failure observed    (workpiece burn or machine or wheel failure)-   Coolant: Master Chemical Trim E210 200, at 10% concentration with    deionized well water, 95 gal/min (360 L/min)-   Workpiece material: AISI 4340 steel 48-50 Rc hardness-   Dress mode: rotary diamond, non-continuous-   Dress compensation: 40 micro-inch/rev (1 micrometer/rev)-   Total radial dress compensation: 0.02 inch/rev (0.5 mm/rev)-   Speed ratio: +0.8

In these grinding runs, the table speed was increased until failure wasobserved. Failure was denoted by workpiece burn or by excessive wheelwear as indicated by power data, wheel wear (WWR) measurements,measurements of surface finish and visual inspection of the groundsurface. The material removal rate (maximum MRR) at which failureoccurred was noted.

As set forth in Table 7-2, below, these grinding tests demonstrated thatthe experimental wheels containing the agglomerates were consistentlyable to achieve higher maximum material removal rates than thecomparative wheels. The experimental wheels also exhibited acceptablevalues for the other, less critical, grinding parameters observed increepfeed operations (i.e., WWR, power and surface finish).

TABLE 7-2 Grinding Test Results Specific Average Maximum GrindingSurface Wheel Composition MRR WWR Energy Roughness Volume % mm³/s/mmmm³/s/mm J/mm³ μm Wheel (agglomerate samples Ex. 2, 5) Agglom.^(a) BondPorosity (10) 37.50 5.70 56.80 16.4 0.27 45.1 1.07 (11) 37.50 5.70 56.8013.6 0.14 45.8 1.04 (12) 37.50 5.70 56.80 16.3 0.43 44.0 1.40 (12) 37.505.70 56.80 13.8 0.14 44.8 1.05 (13) 37.50 5.70 56.80 13.6 0.24 45.8 1.03(27) 37.50 8.40 54.10 16.3 0.21 47.3 0.97 (26) 37.50 8.40 54.10 13.70.17 50.3 0.86 (26) 37.50 8.40 54.10 11.0 0.09 54.4 0.80 (25) 37.50 8.4054.10 13.5 0.12 52.4 0.89 (24) 37.50 8.40 54.10 10.9 0.08 54.6 0.77Comparative samples^(c) non- agglomerated Grain Bond Porosity grain vol% Vol % vol % JOHN02 37.50 4.70 57.80  8.3 0.12 46.7 1.28 38A60-D25VCF2EB030-2 37.50 4.70 57.80 10.8 0.14 46.5 1.16 38A60-D25VCF2 EB012-2 37.505.70 56.80 11.0 0.07 58.5 0.67 38A60-E25VCF2 JOHN01 36.00 4.70 59.3011.0 0.12 54.7 0.68 38A60-D28VCF2 ^(a)At 37.50 vol. % abrasive grain,the comparative wheels contained a greater volume % abrasive grain(i.e., 1-3 volume % more) than the experimental wheels made with 37.50vol. % agglomerated grain, binding material and intra-agglomerateporosity.

EXAMPLE 8

An agglomerated abrasive grain sample (60) was prepared in the rotarycalcination apparatus, with a silicon carbide tube, described in Example1 and illustrated in FIG. 1. The process of agglomeration was carriedout under atmospheric conditions, at 1,350° C., with an apparatus tuberotation rate of 9 rpm, a tube incline angle of 3 degrees, and afeedrate of 6-10 kg/hour.

The agglomerate sample was made from a mixture of 38A alumina abrasivegrain, 60 grit size (same grain as used in Examples 1 and 6), 5.0 wt %binding material F (based on weight of abrasive grain) and 2.5 wt %Binder 3 in water (50/50 weight mixture based on weight of abrasivegrain).

After agglomeration in the rotary calcination apparatus, theagglomerated abrasive grain was screened and tested for loose packingdensity (LPD) and other attributes by the methods described above. Theyield of usable free-flowing agglomerates (defined as -12 mesh to pan)was 72.6% of the feedstock before sintering. The LPD of the agglomeratewas 1.11 g/cc and the relative density was 28.9%. These sinteredagglomerates were used to make grinding wheels having a finished size of16.25×0.75×5.00 inch (41.3×2.4×12.8 cm).

To make the abrasive wheels, the agglomerates were added to a mixeralong with a powdered vitrified bond composition (corresponding toBinding material C from Table 1-2) and liquid Binder 3 to make amixture. The wheels were then molded from this mixture, dried, fired toa maximum temperature of 900° C., graded, finished, balanced andinspected according to commercial grinding wheel manufacturingtechniques known in the art. Wheels were made to correspond in modulusof elasticity value to comparative wheels having a standard wheelhardness grade in the E grade range on the Norton Company hardness gradescale.

The characteristics of the fired abrasive wheels and a comparativecommercial wheel, obtained from Saint-Gobain Abrasives, Inc., Worcester,Mass., are described in Table 8-1 below.

TABLE 8-1 Abrasive Wheels Abrasive Fired Mod. of Wheel Air DensityElasticity Sample Wheel Composition Permeability^(b) g/cc d/cm² × 10¹⁰Agglom. Porosity Bond^(c) Experimental Vol. % vol % vol % 8-4  37.509.88 52.62 90.4 1.66 17.5 8-11 37.5 9.88 52.62 87.4 1.66 17.5 8-17 37.59.88 52.62 88.3 1.66 17.5 Grain Bond Porosity Comparative vol % vol %vol % 38A605-D28VCF2  37.50 5.73 56.77 43.5 1.65 17.3 ^(a)At 37.50 vol.% abrasive grain component, the comparative sample wheels contained alarger volume percentage abrasive grain (i.e., about 1-3 vol. % more)than the experimental wheels of the invention containing a mixture of37.50 vol. % agglomerated grain, binding material and intra-agglomerateporosity. ^(b)Fluid (air) permeability was measured by the test methodsdisclosed in U.S. Pat. Nos. 5,738,696 and 5,738,697, assigned to NortonCompany. Relative air permeability values are expressed incc/second/inch of water units. ^(c)Values for volume % bond do notinclude the volume % binding material used on the grains to make theagglomerates. Volume % bond represents only the materials added to makethe grinding wheels.

Abrasive grinding wheels described in Table 8-1 were tested in acreepfeed grinding test. Parameters for the creepfeed grinding test wereset to yield the following grinding conditions.

Grinding Conditions:

-   Machine: Hauni-Blohm Profimat410-   Mode: Slot creepfeed grind-   Depth of Cut: 0.125 inch (0.318 cm)-   Wheel speed: 5500 surface feet per minute 28 m/sec)-   Table speed: Varied in increments of 2.5 in/min (6.4 cm/min.) from    5-15 inches/minute (12.7-38.1 cm/minute) or until failure observed    (workpiece burn or machine or wheel failure)-   Coolant: Master Chemical Trim E210 200, at 10% concentration with    deionized well water, 95 gal/min ( 360 L/min)-   Workpiece material: AISI 4340 steel 48-50 Rc hardness-   Dress mode: rotary diamond, non-continuous-   Dress compensation: 40 microinch/rev (1 micrometer/rev)-    Total radial dress compensation: 0.02 inch-   Speed ratio: +0.8

In these grinding runs, the table speed was increased until failure wasobserved. Failure was denoted by workpiece burn or by excessive wheelwear as indicated by power data, wheel wear (WWR) measurements andvisual inspection of the ground surface. The material removal rate (MRR)(i.e., the maximum MRR before failure) at which failure occurred wasnoted. Measurements of surface finish also were made.

As set forth in Table 8-2, below, these grinding tests demonstrated thatthe experimental wheels containing the agglomerates were able toconsistently achieve higher material removal rates before burning theworkpiece. The maximum MRR for the comparative wheel was at a tablespeed of only 12.5 inch/minute (5.29 mm/sec), whereas the experimentalwheel's maximum MRR was at a table speed of 15 inch/minute (6.35mm/sec).

The experimental wheels also exhibited comparable, and commerciallyacceptable, values for the other, grinding parameters observed at thehighest MRR achieved by the comparative wheels in this creepfeedoperation (i.e., power and surface finish at the 5.29 mm/sec tablespeed).

TABLE 8-2 Grinding Test Results Table Ave. Work piece Wheel Speed MRRPower Roughness quality Sample mm/s mm³/s, mm W/mm μm observationsexperimental 8-4 3.18 10.00 403.1 0.80 3.18 10.00 411.0 0.80 4.23 13.44516.7 0.89 4.23 13.44 516.7 1.04 5.29 16.77 614.5 0.93 5.29 16.77 638.00.99 maximum 6.35 19.89 712.5 0.88 slight exit burn 8-11 3.18 10.00403.1 0.90 3.18 10.11 395.5 0.86 4.23 14.30 516.7 1.00 4.23 14.09 508.80.93 5.29 16.77 634.1 0.86 5.29 16.67 634.1 0.91 maximum 6.35 19.89724.3 0.97 slight exit burn 8-17 3.18 10.00 411.0 0.99 3.18 10.11 407.20.85 4.23 13.33 528.4 0.94 4.23 13.33 520.5 0.97 5.29 16.67 630.3 0.895.29 16.56 638.0 0.97 maximum 6.35 20.00 716.3 0.99 slight exit burncomparative 2.12 6.77 273.9 0.77 3.18 9.89 391.3 0.79 3.18 10.00 395.50.95 3.18 10.00 399.3 0.93 4.23 13.33 508.8 0.88 4.23 13.44 516.7 0.795.29 16.67 598.9 0.91 severe entry burn 5.29 16.77 618.6 0.83 severeentry burn maximum 5.29 16.77 614.5 0.89 severe entry burn

EXAMPLE 9

Abrasives wheels made with agglomerate sample 35 of Example 5 weretested in a dry surface crossfeed grinding process typical of theprocesses used in toolroom grinding operations. A comparative commercialabrasive wheel was compared to the wheels of the invention in this test.

The abrasive wheels containing agglomerates were made by the method ofExample 8 and fired at a maximum temperature of 900° C., however, thesize of the wheels was 7×0.5×1.25 inch (17.8×1.3×3.2 cm). Fired wheelscontained 40% agglomerates, 11-12.1% vitrified bond and 47.9-49%porosity, on a volume percent basis. Firing conditions for the wheels ofthe invention and properties of the fired abrasive wheels and thecomparative wheels are described in Table 9-1.

TABLE 9-1 Abrasive Wheels Wheel Fired Mod. of (Grade H Air DensityElasticity hardness) Wheel composition Permeability^(b) g/cc GPaExperimental Ex. 5 Agglo. Agglom Bond^(c) Porosity Sample No. vol % vol% vol % 35-1 41.8 11.1 46.9 41.0 1.85 27.2 35-2 42.5 12.1 44.9 31.1 1.9130.8 35-3 40.0 11.0 49.3 58.1 1.80 22.7 Grain Bond Porosity ComparativeWheel^(a) vol % vol % vol % 38A60-H12VBEP 40.5 8.6 50.9 35.7 1.79 26.31330.2 ^(a)At 40.5 vol. % abrasive grain component, the comparativesample wheels contained a larger volume percentage abrasive grain (seeTable 9-2, below) than the experimental wheels of the inventioncontaining 40-42.5 vol. % agglomerated grain (including binding materialand intra-agglomerate porosity). ^(b)Air permeability was measured bythe test methods disclosed in U.S. Pat. Nos. 5,738,696 and 5,738,697,assigned to Norton Company. ^(c)Values for volume % bond do not includethe volume % glass binding material used on the grains to make theagglomerates. Volume % bond represents only the materials added to makethe grinding wheels.

The volume percentage abrasive grain and glass binding material of theagglomerates used in the experimental wheels is set forth in Table 9-2,below.

TABLE 9-2 Wheel composition adjusted for agglomerate components Volume %Volume % Volume bond Sample No. binding % (+binding Volume % Ex. 5Volume % material in grain in material) in porosity in AgglomerateAgglom. Agglom. wheel wheel wheel Experimental 35-1 41.8 3.9 37.9 15.246.9 35-2 42.5 4.0 38.5 16.6 44.9 35-2 40.0 1.9 38.1 12.6 49.3comparative^(a) — — 40.5 8.6 50.9 ^(a)At 40.5 vol. % abrasive grain, thecomparative wheels contained a greater volume % abrasive grain (i.e.,1-3 volume % more) than the experimental wheels made with 40-42.5 vol. %agglomerated grain, binding material and intra-agglomerate porosity.Grinding Conditions:

-   Machine: Brown & Sharpe Surface Grinder-   Mode: Dry surface grind-   Crossfeed: 0.508 mm-   Wheel speed: 3500 rpm; 6500 sfpm-   Table speed: 50 fpm (15240 mm/min)-   Coolant: None-   Workpiece material: D3 steel Rc 60 hardness 203.2 mm long×47.8 mm    wide-   Dress mode: Single point diamond-   Dress Comp.: 0.025-   Dress Lead: 254 mm/min

In these grinding runs, the downfeed was increased until failure wasobserved. In surface tool-room grinding operations, as in creep-feedgrinding operations, the most significant performance parameter is themaximum material removal rate (MRR) capacity of the grinding wheel.Thus, maximum MRR at which grinding failure occurred was noted for eachgrinding wheel, and failure was denoted by visual burn observations ofthe workpiece, excessive power, or by excessive wheel wear rate (WWR).Measurements of surface finish were also made.

As set forth in Tables 9-2 and 9-3, below, this grinding testdemonstrated that the experimental wheels containing the agglomeratesconsistently achieved higher maximum material removal rates before wheelbreakdown by wear. Furthermore, the higher MRRs were achieved with lowerpower while maintaining comparable surface roughness values.

TABLE 9-2 Grinding Test Results Total MRR′ Specific Surface Wheel Infeedmm³/s, G-ratio Energy Finish Sample mm mm MRR/WRR W · s/mm³ Ra (μin)Experimental 35-1 0.102 19.0 9.00 81.9 25 0.152 21.0 7.51 79.6 20 0.20326.1 7.95 64.5 24 0.254 34.2 7.62 55.7 22 0.305 42.9 6.85 44.4 29 0.35650.3 6.89 42.9 19 0.406 51.0 6.39 41.4 30 0.457 64.5 6.86 36.1 21 0.55969.4 5.75 35.9 28 0.660 89.4 6.19 30.0 24 35-2 0.102 17.1 12.82 86.6 230.203 28.1 9.24 62.8 26 0.305 41.9 7.90 51.1 28 0.406 56.8 6.95 40.2 320.508 64.8 5.73 38.1 30 0.610 83.5 5.61 35.1 33 35-3 0.102 12.3 7.13137.5 12 0.203 26.5 8.09 67.9 12 0.305 41.3 7.68 47.7 16 0.406 54.2 6.5441.6 16 0.508 67.1 5.84 34.7 23 comparative 38A60- 0.102 16.5 9.48 98.611 H12VBE 0.203 27.4 8.55 60.9 15 0.305 41.9 6.80 46.6 17 0.406 51.95.92 39.7 18 0.508 52.9 4.02 43.8 25

TABLE 9-3 Grinding Test Results-Wheel wear measurements^(a) % wheelWheel Total Infeed A B C D Area face Sample mm mil mil mil mil mm² wearExperimental 35-1 0.102 0.0033 0.0038 0.1115 0.1424 0.2932 53 0.6600.0151 0.0148 0.2026 0.2283 2.0768 90 35-2 0.102 0.0027 0.0029 0.08790.1149 0.0020 42 0.610 0.0146 0.0149 0.2161 0.2248 2.0982 90 35-3 0.1020.0031 0.0028 0.1083 0.1434 0.2378 53 0.508 0.0119 0.0117 0.1835 0.24021.6110 89 Comparative 38A60-H12VBE 0.102 0.0035 0.0033 0.1117 0.10530.2382 43 0.508 0.0119 0.0115 0.2170 0.2701 1.8350 96 ^(a)Wheel wear wasmeasured by a variation of the method (“corner holding test”) describedin U.S. patent No. 5,401,284, assigned to Norton Company. For the datain this Table, the values A and D were measured at the wheel perimeter,along the wheel grinding face, and the values B and C were measured atequidistant points near the center of the wheel grinding face. Asgrinding progresses, the relative stability of values A and D, comparedto values B and C, is an indicator of the wheel wear resistance of thewheel. The “Area” is the amount of material removed from the wheel. The% wheel face wear reflects the width of the wheel wear at the center ofthe wheel grinding face, near the points where the values B and C aremeasured.

EXAMPLE 10

Abrasives wheels made with abrasive grain agglomerates were tested in aninner diameter (ID) grinding test.

Agglomerates (sample 61) were prepared as described in Example 2, exceptthe temperature was maintained constant at 1170° C. (sample 61).Additionally, a model #KOU-8D48-RTA-20 rotary calciner apparatus,equipped with a 108 inch (274 cm) long, 8 inch (20 cm) inner diameter,silicon carbide tube, having a 48 inch (122 cm) heated length with threetemperature control zones, was used. This apparatus was manufactured byHarper International, Buffalo, N.Y. The process of agglomeration wascarried under atmospheric conditions, with an apparatus tube rotationrate of 6 rpm, a tube incline angle of 2.5-3.0 degrees, and a feed rateof 8-10 kg/hour. The apparatus used was substantially identical to theapparatus illustrated in FIG. 1.

Agglomerate sample 61 was made with 30 lbs (13.63 Kg) abrasive grain(120 grit 32A alumina grain, obtained from Saint-Gobain Ceramics andPlastics, Inc.) and 1.91 lbs (0.87 Kg) binding material A (yielding 6.36wt % binding material in the sintered agglomerate). The binding materialwas dispersed in water (0.9 lbs; 0.41 Kg) prior to addition to thegrain. The agglomerates had an average size of 260 microns and a loosepacked density (LPD) of 1.13 g/cc.

A comparative commercial abrasive wheel was compared to the wheels ofthe invention in this test. The comparative wheel had the same sizedimensions and was made with the same abrasive grain, but withoutagglomerates. The comparative wheel was marked 32A120-LVFL and wasobtained from Saint-Gobain Abrasives, Inc., Worcester, Mass.

To make the experimental abrasive wheel, the agglomerates were added toa mixer along with a powdered vitrified bond composition and liquidBinder 3 to make a mixture. The wheels were then molded from thismixture, dried, fired to a maximum temperature of 900° C., graded,finished, balanced and inspected according to commercial grinding wheelmanufacturing techniques known in the art.

The grinding wheels were 1A type wheels, having a finished-size of1.8×1.0×0.63 inch (4.57×2.54×1.60 cm). The composition andcharacteristics of the experimental and comparative wheels are listedbelow in Table 10-1.

TABLE 10-1 Abrasive Wheels Wheel Mod. of Hard- Fired Elas- ness Densityticity Sample Wheel composition Grade g/cc GPa Experimental AgglomBond^(c) Porosity Wheel vol % vol % vol % 32A120 48 10.26 41.74 L 2.0842.1 Comparative Grain Bond Porosity Wheel^(a,b) vol % vol % vol %32A120 LVFL 52  8.11 39.89 L 2.23 50.9 ^(a)At 52 vol. % abrasive graincomponent, the comparative sample wheels contained a larger volumepercentage grain than the wheels of the invention containing 48 vol % ofa mixture of agglomerated grain with binding material. After deductingthe percent of binding material, the experimental wheel contains only43.4 volume % of grain, 8.6 volume % less grain than the comparativestandard wheel of the same grade. ^(b)Abrasive grain grit size of 120corresponds to 142 microns. ^(c)Values for volume % bond do not includethe volume % binding material used on the grains to make theagglomerates. Volume % bond represents only the materials added to makethe grinding wheels.

Abrasive grinding wheels described in Table 10-1 were tested in an innerdiameter (ID) grinding test. Parameters for the ID grinding test wereset to yield the following grinding conditions.

Grinding Conditions:

-   Machine: Okuma ID grinder-   Mode: Wet ID, plunge, climb grind-   Wheel speed: 18000 rpm-   Work speed: 600 rpm-   Coolant: Master Chemical Trim E210, 5% in deionized well water-   Workpiece material: 52100 steel Rc 60 hardness rings: 2.225×0.50    inch (5.65×1.28 cm)-   Dress mode: Rotary single point diamond-   Dress Ratio.: 0.650-   Dress Lead: 0.304 mm/rev

In these tests, three sets of grinds were conducted at constant infeedrates and five grinds were conducted for each set. The infeed rate setsa nominal material removal rate for each test. In ID grindingoperations, the most significant performance parameters are the G-ratio(MRR/wheel wear rate (WWR)), the specific energy required to grind at aset infeed rate and the resulting surface finish. Data in the tablebelow is given for each set of infeed rates; the surface finish datarepresents the value after the fifth grind of each set.

As set forth in Table 10-2, below, these grinding tests demonstrated theperformance of the experimental wheel containing the agglomerates wascomparable to, or better than, that of the comparative wheel in G-ratio(MRR/wheel wear rate (WWR)), specific grinding energy and surfacefinish. These results are surprising in view of the significantly lowervolume percentage of abrasive grain in the experimental wheel. Withinnormal wheel structures, the volume % abrasive grain is the mostsignificant variable in determining the G-ratio. In the absence of othervariables, a higher grain content results in a proportionally higherG-ratio. A reduction in the volume percentage of grain needed to achievethe same or better G-ratio represents a significant technicalimprovement in the abrasive tool.

TABLE 10-2 Grinding Test Results Radial Specific Infeed MRR G-ratio^(a)Grinding Surface Wheel Rate mm³/s, WWR/ Energy Finish Sample mm/min mmMRR J/mm³ Ra comparative 32A120 1.10 3.25 50.5 52.1 0.72 LVFL 1.83 5.4559.4 49.4 0.84 2.54 7.66 42.5 49.1 1.19 experimental 32A120 1.10 3.2565.8 (78.8) 52.1 0.82 1.83 5.45 55.0 (65.9) 48.3 1.02 2.54 7.66 42.9(51.4) 45.9 1.18 ^(a)The G-ratio given in parentheses for theexperimental wheel is a value adjusted for the smaller volume percentageof abrasive grain in the experimental wheel. In other words, thevolumepercentage of grain in the experimental wheels is only 83.46% of thevolume percentage of grain in the comparative wheels. Thus, theexperimental wheel G-ratio values shown in the parentheses have beennormalized to the volume % grain of the comparative wheels in order toobtain a performance measure based upon total abrasive grain usage.

EXAMPLE 11

The agglomerated abrasive grain of the invention was used to manufacturelarge abrasive wheels in order to confirm the feasibility ofmanufacturing such wheels without the use of added pore inducers andusing such wheels in creepfeed grinding.

The agglomerated abrasive grain (sample 62) was prepared in the rotarycalcination apparatus, with a silicon carbide tube, described in Example1 and illustrated in FIG. 1. The process of agglomeration was carriedout under atmospheric conditions, at 1,350° C., with an apparatus tuberotation rate of 9 rpm, a tube incline angle of 3 degrees, and afeedrate of 6-10 kg/hour.

The agglomerate grain sample 62 was made from a 50/50 mixture of 32A and38A alumina abrasive grain, both 60 grit size (same grain as used inExamples 1 and 6), 5.0 wt % binding material E (based on weight ofabrasive grain) and 2.5 wt % Binder 3 (50/50 weight mixture in waterbased on weight of abrasive grain).

After agglomeration in the rotary calcination apparatus, theagglomerated abrasive grain samples were screened and tested for loosepacking density (LPD) and other attributes by the methods describedabove. The yield of usable free-flowing granules (defined as -12 mesh topan) was 74.1% of the total weight of the feedstock before calcination.The LPD of the agglomerate was 1.14 g/cc, and the relative density was30.0%.

These sintered agglomerates were used to make relatively large (e.g., 20inch (50.8 cm) diameter) creep-feed grinding wheels. Comparative wheelsof this size normally are made with bubble alumina or other solid orclosed cell pore inducers as aids to stiffen the structure and preventwheel shape distortion from slumpage during firing as the vitrified bondmelts and flows. Bubble alumina is particularly effective in preventingslumpage but it is undesirable in grinding performance as it createsclosed cell porosity.

To make the experimental abrasive wheels, the agglomerates were added toa mixer along with a powdered vitrified bond composition (correspondingto Binding material C from Table 2) and liquid Binder 3 to make amixture. The wheels were then molded from this mixture, dried, fired toa maximum temperature of 900° C., graded, finished, balanced andinspected according to commercial grinding wheel manufacturingtechniques known in the art. The fired wheels were then finished to asize of 20×1×8 inch (50.8×2.5×20.3 cm). A moderate, but commerciallyacceptable degree of slumping of the experimental wheels was observedduring firing of the wheels.

Wheels were designed to correspond in volume percentage composition anddensity to comparative, commercial wheels having a standard wheelhardness grade between the C and D grades on the Norton Company hardnessgrade scale.

The characteristics of the finished experimental and comparativeabrasive grinding wheels are described in Table 11-1 below. Althoughwheel composition percentages and densities would have predicted wheelshaving equivalent wheel hardness values, in fact, the modulus ofelasticity confirmed the experimental wheels were of a softer grade thanthe comparative wheels. The air permeability values show the porosity ofthe experimental wheel, in contrast to that of the comparative wheel, tobe porosity having open permeability, permitting free flow of coolant inthe wheel and easy removal of grinding debris from the grinding face ofthe wheel.

TABLE 11-1 Abrasive Wheels Relative Fired Mod. of Air Density ElasticityWheel Sample Wheel Composition Permeability^(b) g/cc d/cm² × 10¹⁰Experimental Agglom. Bond Porosity Agglomerate Vol. % vol % vol % 6236.00 7.03 56.97 74.9 1.52 10.24 Grain Bond Porosity Comparative vol %vol % vol % 32A605-D28VCF2 36.00 5.50 58.50 46.2 1.52 14.01 ^(a)At 36.0vol. % abrasive grain component, the comparative sample wheels containeda larger volume percentage grain (i.e., about 1-2 volume % more) thanthe wheels of the invention containing a mixture of 36.0 vol. % of acombination of agglomerated grain and binding material. ^(b)Fluid (air)permeability was measured by the test methods disclosed in US Pat. Nos.5,738,696 and 5,738,697, assigned to Norton Company. Relative airpermeability values are expressed in cc/second/inch of water units.

The wheels were tested in the creepfeed grinding operation described inExample 7 along with the comparative creepfeed grinding wheel describedin Table 11-2. The comparative wheel was a standard commercial productavailable from Saint-Gobain Abrasives, Inc., Worcester, Mass. It had thesame size dimensions and was otherwise comparable to the experimentalwheels, but had been made with bubble alumina filler, and no abrasivegrain agglomerates.

TABLE 11-2 Grinding Test Results Table Specific Wheel Speed MRR EnergySample mm/s mm³/s, mm J/mm³ Comparative 2.1 6.7 56.6 3.2 10.0 47.0 5.316.5 39.2 Experimental 2.1 6.7 55.7 3.2 10.0 46.5 5.3 16.7 40.0

These results demonstrate the feasibility of manufacturing and using acreepfeed grinding wheel of the dimensions tested without the use of aclosed porosity filler material such as bubble alumina.

EXAMPLE 12

The agglomerate size distribution was compared before and after moldingabrasive grinding wheels of the invention to examine the integrity andstrength of the agglomerates in abrasive wheel manufacturing processes.The agglomerate size distribution was then compared with the abrasivegrain size distribution of the grain used to make the agglomerates toconfirm that the agglomerates still comprised a plurality of abrasivegrains after molding grinding wheels.

Agglomerates (sample nos. 63, 64, 65) were prepared as described inExample 2, except the temperature was maintained constant at 1200° C.(for samples 63 and 64) or at 1300° C. (sample 65). Additionally, arotary calciner apparatus (model Bartlett-Snow™), manufactured by AlstomPower, Naperville, Ill., equipped with a 120 inch (305 cm) long, 6.5inch (16.5 cm) inner diameter, proprietary high temperature metal alloytube, having a 72 inch (183 cm) heated length with four temperaturecontrol zones, was used. The process of agglomeration was carried underatmospheric conditions, with an apparatus tube rotation rate of 9 rpm, atube incline angle of 2.5 degrees, and a feed rate of 10-14 kg/hour. Theapparatus used was substantially identical to the apparatus illustratedin FIG. 1.

Agglomerate samples 63, 64, and 65 were made with abrasive grainsobtained from Saint-Gobain Ceramics and Plastics, Inc. and differentbinding materials as described in Table 12-1 below.

TABLE 12-1 Agglomerate Compositions Abrasive grain wt % mixture Wt %Sample grit size Binding Binding No. grain type Material Material 6370/30 wt % C 4.5 46 grit 86A alumina/ 46 grit Norton SG ® sol gelalumina 64 50/50 wt % C 4.5 46 grit 38A alumina/ 46 grit Norton SG ® solgel alumina 65 46 grit 55A alumina A 4.5

Experimental wheels were mixed and molded to the size and shapedescribed in Example 10, using a powdered vitrified bond composition andliquid Binder 3. The bond composition used for wheels containingagglomerates 63 and 64 corresponded to Binding Material C, and forwheels containing agglomerate 65 corresponded to Binding Material E,described in Table 2. The volume % of agglomerates, bond and porosityare described in Table 12-2 below.

After molding the wheels under pressure to obtain a “green” wheel andprior to firing these molded wheels, the wheel bond materials werewashed out of the green wheel structure under running water and theagglomerates and abrasive grain were recovered. The size of therecovered agglomerates and grain was determined by screening themthrough a series of U.S. sieve size mesh screens and measuring theweight fraction for each screen. Results are shown in Table 12-2, below,for wheels made to three different specifications.

TABLE 12-2 Size distribution of agglomerates following wheel moldingAve. Ave. Ave. Agglom. Size Initial Initial Initial Size Size afterDistribution Grain Agglom. Distribution molding & of Molded Wheel Vol %Vol % Vol % Size Size Agglom. washing Agglom. Sample aggl. bond pores μmμm Range μm μm Range μm 12-1 40 11.55 48.45 355 998 500-1700 824355-1200 12-2 40 11.55 48.45 355 920 500-1700 767 355-1200 12-3 40 8.551.50 355 1035 500-1700 863 355-1200

The data of Table 12-2 demonstrate from the average dimensions of thesintered agglomerates (before and after processing) that a plurality ofabrasive grains has been retained in the sintered agglomerates afterthey have been molded to form a grinding wheel. While the initial sizeof the agglomerates has been reduced by a minor percentage (e.g., a dropfrom 998 to 824 μm, or a 17% reduction, for sample 12-1), the majorityof the agglomerates have retained their initial size.

The distribution of weight fractions after screening each sample isgiven in Tables 12-2a, 12-2b and 12-2c, below for samples 12-1, 12-2 and12-3, respectively.

TABLE 12-2a Particle size distributions for Sample 12-1 Sieve # Weight %on Screen ISO 565 Initial size of agglomerate Agglomerate Sieve #opening Initial grit size size size distribution ASTM-E μm distributiondistribution after molding 70 212 0 60 250 5 50 300 28 45 355 53 5.7 40425 14 2.9 35 500 1.1 6.0 30 600 0 3.4 11.1 25 725 8.7 15.8 20 850 18.221.2 18 1000 29.0 20.9 16 1180 37.9 16.5 −10/+12 1700 0.9 0

The data in Table 12-2a shows that the largest single grains in the sizedistribution of the initial grit sample are 425 μm in size. The initialagglomerate size distribution data shows that all agglomerates arelarger than 425 μm. After molding and washing, the retained, pressedagglomerates are all larger than 300 μm, and 91.4 wt. % of theagglomerates are larger than the largest single grain (425 μm),confirming the retention of plurality of grains in the sinteredagglomerates after molding a grinding wheel comprising the agglomerates.

TABLE 12-2b Particle size distributions for Sample 12-2 Sieve # Weight %on Screen ISO 565 Initial size of agglomerate Agglomerate Sieve #opening Initial grit size size size distribution ASTM-E μm distributiondistribution after molding 70 212 0 60 250 5 50 300 28 0 45 355 53 0 6.340 425 14 0.2 2.3 35 500 1.0 6.2 30 600 0 5.4 14.1 25 725 15.1 21.9 20850 28.3 25.8 18 1000 31.2 17.3 16 1180 18.8 6.0 −10/+12 1700 0 0

The data in Table 12-2b shows that the largest single grains in the sizedistribution of the initial grit sample are 425 μm in size. The initialagglomerate size distribution data shows that 99.8 wt. % of theagglomerates are larger than 425 μm. After molding and washing, theretained, pressed agglomerates are all larger than 300 μm and 91.4 wt. %of the agglomerates are larger than the largest single grain (425 μm),confirming the retention of plurality of grains after molding.

TABLE 12-2c Particle size distributions for Sample 12-3 Sieve # Weight %on Screen ISO 565 Initial size of agglomerate Agglomerate Sieve #opening Initial grit size size size distribution ASTM-E μm distributiondistribution after molding 70 212 0 60 250 5 50 300 28 0 45 355 53 0 7.240 425 14 2.5 2.9 35 500 1.3 5.1 30 600 0 2.7 8.5 25 725 5.8 11.8 20 85012.3 17.2 18 1000 24.3 21.5 16 1180 49.1 25.8 −10/+12 1700 1.9 0

The data in Table 12-2c shows that the largest single grains in the sizedistribution of the initial grit sample are 425 μm in size. The initialagglomerate size distribution data shows that 97.5 wt. % of theagglomerates are larger than 425 μm. After molding and washing, theretained, pressed agglomerates are all larger than 300 μm, and 89.9 wt.% of the agglomerates are larger than the largest single grain (425 μm),confirming the retention of plurality of grains after molding.

These results demonstrate that agglomerates made according to theinvention have sufficient strength to withstand commercial abrasivewheel molding and handling operations. The abrasive grains present inthe molded wheel retain a three-dimensional structure characteristic ofthe initial abrasive grain agglomerates. A major percentage (i.e., atleast 85 weight %) of the agglomerates retain a plurality of abrasivegrains held in a three-dimensional shape of approximately the same sizeas the initial size of the sintered agglomerates after handling andmolding.

EXAMPLE 13

The structures of abrasive grinding wheels made with the agglomerates ofthe invention were compared under a scanning electron microscope to thestructures of comparative grinding wheels. The comparative wheels weremade without the agglomerates, but comprising the same abrasive grainand bond materials in the same volume percentages of grain, bond andporosity as the grinding wheels of the invention.

Agglomerates (sample no. 66) were prepared as described in Example 10,except the temperature was maintained constant at 1150° C.

Agglomerate sample 66 was made with 150 lbs (68.04 Kg) abrasive grain(80 grit 32A alumina grain, obtained from Saint-Gobain Ceramics andPlastics, Inc.) and 10.23 lbs (4.64 Kg) Binding Material C (yielding6.82 wt % binding material in the sintered agglomerate). The BindingMaterial was dispersed in Binder 3 (3.75 lbs; 1.701 Kg) prior toaddition to the grain.

Experimental wheels were made as described in Example 10 fromagglomerate sample 66. Comparative commercial wheels marked as32A80L8VFL, obtained from Saint-Gobain Abrasives, Inc., were selectedfor comparison.

A photograph of a cross-section of each wheel was taken at amagnification of 40×. These photographs are shown in FIG. 2(experimental wheel with agglomerates) and 3 (comparative wheel withoutagglomerates). It can be seen that the agglomerates and the pores areirregularly and randomly shaped and sized. The comparative wheel has amuch more ordered and regular structure. It is possible to observe twotypes of pores in the wheels made with the agglomerates:intra-agglomerate pores and larger inter-agglomerate pores appearing asdistinct channels between agglomerates. From permeability testing of theexperimental wheels it has been established that the inter-agglomeratepores are interconnected and render the entire wheel permeable tofluids. Thus, the abrasive grinding wheels of the invention exhibit aporosity that includes a major amount of interconnected porosity (i.e.,at least 30 volume % interconnected porosity) and, preferably, a bimodalporosity distribution. The abrasive grinding wheels of the invention arecharacterized by a much more open composite structure than conventionalgrinding wheels.

As can be observed from FIGS. 2 and 3, the maximum dimension of theinter-agglomerate pores is about 2-20 times larger than the maximumdimension of the intra-agglomerate pores. The exact ratio of pore sizedepends upon the composition of the wheels. The ratio of 2-20 applies tothese wheels made with a range of about 8-10 volume percent bond, and anaverage abrasive grain size of about 260 microns. In general, for theabrasive wheels of the invention, as the volume percentage bondincreases from this range, the intra-agglomerate pores become smaller,but the inter-agglomerate pores retain a maximum dimension roughlyequivalent to the maximum dimension of the abrasive grain used in theagglomerates. As the volume percentage bond decreases from this range,the intra-agglomerate pores become relatively larger, but theinter-agglomerate pores retain a maximum dimension roughly equivalent tothe maximum dimension of the abrasive grain used in the agglomerates.

In further microscopic examinations of the wheels made withagglomerates, particularly with agglomerates containing at least 6weight % binding material, it has been observed that increasing theweight percentage of added bond material results in a wheel structurehaving much smaller intra-agglomerate pores. For example, with a higherbinding material weight % and a higher bond volume %, the size ratio canbe about 20-200 times larger for the inter-agglomerate pores than forthe intra-agglomerate pores. It is believed the bond material added tothe agglomerates is drawn into the interstitial area of the agglomeratesduring mixing, molding and thermal processing of the wheels, therebynarrowing or closing off some of the intra-agglomerate porosity andeventually causing a loss of bimodal pore distribution.

EXAMPLE 14

Sintered agglomerates were prepared by a batch oven method from thematerials described in Table 14-1. The abrasive grain was 100 grit(0.173 mm) size 38A alumina grain, obtained from Saint-Gobain Ceramics &Plastics, Inc., Worcester, Mass.

TABLE 14-1 Sintered Agglomerate Composition Weight % of Weight % ofMaterials Pre-fired Mixture Agglomerate Binding Material A 2.85 3.0Binder 1.46 0.0 Walnut Shell Particles 4.34 0.0 38A Abrasive Grain 91.3597.0 Total 100.00 100.0

In the first step of forming the agglomerate particles, the abrasivegrain and walnut shell particles were blended in a Hobart® mixer(laboratory Model N-50). This blend was subsequently wetted with aneffective amount of organic liquid binder (a mixture of 40 wt % liquidanimal glue, 30 wt % powdered maleic acid and 30 wt % water) to adherethe binding material powder to the grain. After wetting these particles,a powder mixture containing the binding material components (a vitrifiedbond composition having the fired composition shown above as “Bindingmaterial A”) was added and mixed. The binding material adhered to thewetted particles and this mixture was then loosely spread over a ceramicfiring batt.

The mixture was fired at 1230° C. for four hours in an electric kiln.After firing, the sintered agglomerates were obtained from the firedmixture by crushing the mixture in a mortar with a pestle. The sinteredagglomerates were sized into three sizes with U.S. standard testingsieves mounted on a vibrating screening apparatus (Ro-Tap; Model RX-29;W.S. Tyler Inc. Mentor, Ohio). The loose packed density of the sinteredagglomerates (LPD) was measured by the American National Standardprocedure for Bulk Density of Abrasive Grains.

After the sizing process, the sintered agglomerates hadthree-dimensional shapes (varying among triangular, cubic, rectangularand various other geometric shapes) and were of the size and LPD shownin Table 14-2.

TABLE 14-2 Sized Sintered Agglomerates Sintered Agglomerate ApproximateLPD Sample Grit Size Size in mm (FEPA) g/cc 14-1 −40/+50 mesh 1.12 (46)0.300-0.425 (300-425 μm) 14-2 −50/+60   1.33 (54-60) 0.250-0.300(250-300 μm)  14-33 −30/+40 0.94 (36) 0.425-0.600 (425-600 μm)

Additional agglomerates were made by slight variations of this process.The variations included the following. The prepared mixture waswet-screened through box screens (8 to 12 mesh) onto trays. The screenedmaterial was then air or oven dried. The material was loaded intoceramic batts. The ceramic batts containing the material were fired inperiodic or tunnel kilns under firing conditions ranging from 1225 to1280 degrees C. for times ranging from 30 to 360 minutes. The firedmaterial was removed from the ceramic batts and processed through a rollcrusher to break up the material into agglomerates.

The crushed material was sized to the desired range using a Ro-Tapapparatus.

Abrasive Wheels

The finished wheels were 3.0×0.525×1.25 inches (7.6×1.34×3.2 cm) insize. The composition of the wheels (volume % of the fired wheels),density, air permeability, grade and modulus properties of the wheelsare described in Table 14-3.

TABLE 14-3 Abrasive Wheels Wheel Sample Ex. 1 Fired Mod. of (agglomerateAgglom Bond Vol % Porosity Relative Air Density Elasticity sample^(a))Vol % Bond B Vol. % Permeability^(b) g/cc d/cm² × 10¹⁰ Grade 14-1 36 6.457.6 n/a 1.577 14.3 D 14-2 36 6.4 57.6 51.0 1.673 20.7 F 14-3 40 6.453.6 n/a 1.831 28.4 H comparative 0.0 5.5 58.5 28.5 1.564 12.9 D sample14-C1 (grain = 36 vol %) ^(a)Agglomerates contained 97 wt % 100 grit 38Aalumina grain and 3 wt % Binding material A and were screened to aparticle size of −40/+60 mesh (250 to 425 μm). ^(b)Fluid (air)permeability was measured by the test methods disclosed in US Pat. Nos.5,738,696 and 5,738,697, assigned to Norton Company. Relative airpermeability values are expressed in cc/second/inch of water units. (A2.2 size nozzle was used on the apparatus). c. At 36 vol. % abrasivegrain, the comparative wheels contained a greater volume % abrasivegrain (i.e., 1-3 volume % more) than the experimental wheels made with36-40 vol. % agglomerated grain, binding material and intra-agglomerateporosity.

The bond used for wheel samples 1, 2 and 3 of the invention was avitrified bond material having the fired molar composition of Bindingmaterial B of Table 2, above. The bond used in the comparative wheelsample had the fired molar composition of Binding material A of Table 2.

The sintered agglomerates and bond mixture of samples 1, 2 and 3 of theinvention were dry blended in a Hobart mixer, filled into molds, coldpressed and fired at a maximum temperature of 735° C. for 4 hours toform the grinding wheel.

The comparative wheel sample was made by blending the vitrified bondcomponents with the abrasive grain in a Hobart mixer. The abrasive grainused in the comparative sample was a 38A alumina grain, 100 grit size(125 μm), obtained from Saint-Gobain Ceramics & Plastics, Inc.,Worcester, Mass. After blending, the mix was molded, pressed and firedat 1230° C. for 4 hours to form the grinding wheel.

Grinding Test 14-A

The wheels of the invention and the comparative wheels were tested in aninternal diameter, creepfeed grinding test using the followingconditions.

Grinding Conditions:

-   Machine: Heald CF, OD/ID Grinder-   Mode: Internal diameter (ID) creepfeed grind-   Wheel speed: 6,319 rpm; 4,968 surface feet per minute (25 M/sec)-   Work speed: 20 rpm-   Grinding mode: ID climb plunge-   Infeed rate: 0.025 inch (0.64 mm)/0.050 inch (1.27 mm) on diameter-   Coolant: Trim E210, 5% ratio with deionized well water, 9 gal/min    (34 L/min)-   Workpiece material: 52100 Steel 4 inch (10.2 cm) ID×0.250 inch    (1cm), Rc-62.0 hardness-   Rotary dress: AX1440, comp. 0.0005 inch, 0.005 inch lead, 2600 rpm

In these grinding runs, the maximum material removal rates (MRR) at theinitial workpiece burn (or initial wheel failure) were measured and theresults observed. Results of these grinding tests are shown in Table14-4.

TABLE 14-4 Grinding Test Results Specific MRR G-ratio EnergyGrindability Sample mm³/s, mm MRR/WWR W · s/mm³ mm³/W · s comparative1.288 81.0 40 2.59 wheel 2.482 40.4 67 1.50 4.544 24.3 113 0.97 Max. MRR5.662 2.9 123 0.13 14-1 1.247 90.9 42 2.73 2.534 85.5 69 3.12 4.870 37.3110 1.66 Max. MRR 6.680 5.7 145 0.26 14-2 2.554 113.7 69 4.19 wheel4.921 76.1 131 2.86 8.061 34.1 208 1.32 Max. MRR 11.116 10.9 265 0.4614-3 2.483 122.3 78 3.89 wheel 5.111 79.4 132 3.07 8.534 34.5 265 1.11Max. MRR 11.545 10.0 340 0.34

The results show the grinding wheels made according to the inventionwere superior in MRR to the closest comparative grinding wheels, and thesuperior performance did not cause excessive power draw (specific energyW.s/mm³) or damage to the surface of the workpiece. Experimental wheelsalso showed G-ratio and grindability index improvements. Furthermore,the grit size of the grain used in the sintered agglomerates of thewheels of the invention was smaller than the grit size of the grain usedin the comparative wheel. All other variables being equal, smaller gritsize yields inferior G-ratio and grindability index. Thus, the superiorperformance of the inventive wheels is significant and unexpected.

Grinding Test 14-B

A second series of grinding runs was conducted with the same group ofwheel samples under the following surface grinding conditions using 4340steel as the workpiece.

Grinding Conditions:

-   Machine: Brown & Sharp Micr-a-size Grinder-   Mode; Surface creepfeed grind-   Wheel speed: 6,000 rpm-   Table speed: 0-   Downfeed: 1.270 mm-   Infeed: 1.270 mm-   Coolant: Trim VHPE 210, 1:20 ratio with deionized well water, 9    gal/min (34 L/min)-   Workpiece material: 4340 Steel; 51 Rc hardness; 95.4 mm length;    203.2 mm width-   Dressing: single point diamond tool, comp. 0.025 mm, speed 254    mm/min

TABLE 14-5 Grinding Test Results (Average of Multiple Runs) SpecificSample MRR G-ratio Energy Grindability (run) mm³/s, mm WWR/MRR W · s/mm³mm³/W · s 14-C1 comparative wheel 1 3.032 * 49.46 * 2 4.500 54.1 41.31.311 3 7.597 10.5 72.53 0.144 14-1 wheel 1 3.045 32.7 51.61 0.635 24.510 23.2 82.50 0.281 3 7.597 33.4 32.00 1.045 14-2 wheel 1 2.987 160.857.86 2.780 2 4.548 163.9 40.53 4.043 3 7.597 83.4 30.34 2.750 14-3wheel 1 3.052 27.4 52.34 0.523 2 4.577 164.9 53.73 3.069 3 7.742 10.756.11 0.190 * G-ratio and grindability could not be measured for thisrun.

The results show the grinding wheels made according to the inventionwere superior in G-ratio and grindability index to the closestcomparative grinding wheels, and the superior performance did not causeexcessive power draw or damage to the surface of the workpiece.

EXAMPLE 15

Additional abrasive wheels were made from sintered agglomerates preparedaccording to the method of Example 14, except different types ofabrasive grains and binding materials were used in the sinteredagglomerate samples. The compositions of the agglomerates and of theabrasive wheels are set forth in Table 15-1. In the wheels of theinvention, the vitrified bond materials were selected to have a meltingtemperature at least 150° C. higher than the melting temperature of thebinding materials in the agglomerates used to make the wheels.

All sintered agglomerates contained 3 wt % binding material and 97 wt %grain and were screened to a particle size of −20/+45 mesh (US standardsieve size) (355 to 850 μm).

The finished wheels were 7.0×0.50×1.25 inches (17.8×1.27×3.2 cm) insize. The composition of the wheels (volume % of the fired wheels),density, and modulus properties of the wheels are described in Table15-1.

The bond for the experimental wheels had the molar composition ofBinding material B of Table 2 and the wheels made with this bond werefired at 735° C. for 4 hours. The comparative wheels were made with avitrified bond having the molar composition of Binding material C ofTable 2 and these wheels were fired at 900° C. for 8 hours. Comparativewheels made without sintered agglomerates contained 40 vol % abrasivegrain and either 10.26 vol % (H grade hardness) or 6.41 vol % (F gradehardness) vitrified bond.

TABLE 15-1 Agglomerates and Abrasive Wheels Experimental AgglomerateFired Mod. of Wheel Sample Grain Grit Size Agglomerate Bond PorosityRelative Air Density Elasticity (Grade) Binding material Vol % Vol %Vol. % Permeability^(b) g/cc d/cm² × 10¹⁰ 15-1 32A-II 60 grit 40 10.349.7 34.4 1.847 27.8 (H) Binding material A 15-2 Alomax ® 60 grit 4010.3 49.7 33.4 1.835 27.3 (H) Binding material A 15-3 Norton SG ® 60grit 40 10.3 49.7 23.3 1.850 29.6 (H) Binding material D 15-4 Norton SG60 grit 40 6.4 53.6 46.5 1.730 20.9 (F) Binding material DComparative^(a) Abrasive grain type samples grain = 40 vol % 15-C1Norton SG 60 grit 0.0 10.3 49.7 16.6 1.818 31.6 (H) 15-C2 Norton SG 60grit 0.0 6.4 53.6 35.1 1.715 22.1 (F) 15-C3 Norton SG 46 grit 0.0 10.349.7 16.0 1.822 32.6 (H) 15-C4 Norton SG 60 grit 0.0 6.4 53.6 41.9 1.73623.1 (F) 15-C5 32A-II 60 grit 0.0 10.3 49.7 15.0 1.832 32.5 (H) 15-C6Alomax 60 grit 0.0 10.3 49.7 16.0 1.837 31.9 (H) ^(a)At 40 vol. %abrasive grain, the comparative wheels contained a greater volume %abrasive grain (i.e., about 2-3 volume % more) than the experimentalwheels made with 40 vol. % agglomerated grain, binding material andintra-agglomerate porosity. ^(b)Fluid (air) permeability was measured bythe test methods disclosed in US Pat. Nos. 5,738,696 and 5,738,697,assigned to Norton Company. Relative air permeability values areexpressed in cc/second/inch of water units. (A 2.2 size nozzle wasused).

The properties of these wheels, especially the air permeability valueswithin a single wheel grade, demonstrate a higher degree ofinterconnected porosity in the structures of the experimental wheelsmade from agglomerated abrasive grain than in comparative wheels made tothe same volume percent porosity and grade with the same grain and bondmaterials. This structural difference has been observed in differentwheel hardness grades, with different types of grain and bond and fordifferent volume percentages of abrasive wheel components.

1. A bonded abrasive tool, having a structure permeable to fluid flow,the tool comprising: a) about 34-56 volume % non-elongated abrasivegrain; b) about 3-25 volume % bond; and c) about 35-80 volume % totalporosity, including at least 30 volume % interconnected porosity,wherein pores included in the total porosity are irregularly andrandomly shaped and sized; the bonded abrasive tool being substantiallyfree of porosity inducing media and elongated shaped materials having alength to cross-sectional width aspect ratio of at least 5:1; wherein5-100 volume % of the abrasive grain comprises abrasive grain heldwithin sintered agglomerates, and the sintered agglomerates comprise aplurality of abrasive grains with a binding material, and the sinteredagglomerates have an average diameter that is no greater than an averagedimension of the interconnected porosity when the interconnectedporosity is measured at a point of a maximum opening.
 2. The bondedabrasive tool of claim 1 wherein the bond is a vitrified bond.
 3. Thebonded abrasive tool of claim 1 wherein the tool has a maximum densityof 2.2 g/cc.
 4. The bonded abrasive tool of claim 1 wherein the bindingmaterial comprises at least one of ceramic materials, vitrifiedmaterials, and vitrified bond compositions.
 5. The bonded abrasive toolof claim 1 wherein the sintered agglomerates are 200 to 3,000micrometers in average diameter.
 6. The bonded abrasive tool of claim 1wherein the abrasive grains are microabrasive grains and the sinteredagglomerates are 5 to 180 micrometers in average diameter.
 7. The bondedabrasive tool of claim 1 wherein the bonded abrasive tool has a bimodalporosity distribution of intra-agglomerate pores and interconnectedporosity.
 8. The bonded abrasive tool of claim 1 wherein the toolfurther comprises at least one of a secondary abrasive grain, fillermaterials, and grinding aids.
 9. The bonded abrasive tool of claim 1wherein the tool comprises 36 to 52 volume % abrasive grain, 3 to 13volume % bond and 35 to 70 volume % porosity.
 10. The bonded abrasivetool of claim 1 wherein the bond is selected from the group consistingof organic bonds and metal bonds.
 11. The bonded abrasive tool of claim1 wherein the binding material has a melting temperature between about500 and 1400° C.
 12. The bonded abrasive tool of claim 1 wherein thebinding material is a ceramic material selected from silica, alkali,alkaline-earth, mixed alkali and alkaline-earth silicates, aluminumsilicates, zirconium silicates, hydrated silicates, aluminates, oxides,nitrides, oxynitrides, carbides, oxycarbides and combinations andderivatives thereof.
 13. The bonded abrasive tool of claim 1 wherein thesintered agglomerates have an average size dimension two to twenty timeslarger than the average size of the abrasive grain.
 14. The bondedabrasive tool of claim 1 wherein at least 50%, by weight, of thesintered agglomerates are 200 to 3,000 micrometers in average diameter.15. The bonded abrasive tool of claim 1 wherein the abrasive grains aremicroabrasive grains and at least 50%, by weight, of the sinteredagglomerates are 5 to 180 micrometers in average diameter.
 16. Thebonded abrasive tool of claim 1 wherein the tool further comprises atleast one of secondary abrasive grain, filler materials, and grindingaids.
 17. A method of grinding comprising the stops of: a) providing abonded abrasive tool, having a structure permeable to fluid flow, thetool comprising: 1) about 34-56 volume % non-elongated abrasive grain;2) about 3-25 volume % bond; and 3) about 35-80 volume % total porosity,including at least 30 volume % interconnected porosity, wherein poresincluded in the total porosity are irregularly and randomly shaped andsized; the bonded abrasive tool being substantially free of porosityinducing filler material and elongated shaped materials having an aspectratio of at least 5:1, wherein 5-100 volume % of the abrasive graincomprises abrasive grain held within sintered agglomerates, and thesintered agglomerates comprise a plurality of abrasive grains with abinding material, and the sintered agglomerates have an average diameterthat is no greater than an average dimension of the interconnectedporosity when the interconnected porosity is measured at a point of amaximum opening; b) bringing the bonded abrasive tool into contact witha workpiece; and c) abrading the surface of the workpiece with thebonded abrasive tool.
 18. A bonded abrasive tool comprising: a) about34-56 volume % non-elongated abrasive grain; b) about 3-25 volume %bond; and c) about 35-80 volume % total porosity, including at least 30volume % interconnected porosity, wherein pores included in the totalporosity are irregularly and randomly shaped and sized; the bondedabrasive tool being substantially free of elongated shaped materialshaving a length to cross-sectional width aspect ratio of at least 5:1;wherein 5-100 volume % of the abrasive grain is held within sinteredagglomerates, and the sintered agglomerates have an average diameterthat is no greater than an average dimension of the interconnectedporosity when the interconnected porosity is measured at a point of amaximum opening.
 19. The bonded abrasive tool of claim 18 wherein thesintered agglomerates are 200 to 3,000 micrometers in average diameter.20. The bonded abrasive tool of claim 18 wherein the abrasive grains aremicroabrasive grains and the sintered agglomerates are 5 to 180micrometers in average diameter.
 21. The bonded abrasive tool of claim18 wherein the bonded abrasive tool has a bimodal porosity distributionof intra-agglomerate pores and interconnected porosity.
 22. A bondedabrasive tool comprising: a) about 34-56 volume % non-elongated abrasivegrain; wherein 5-100 volume % of the abrasive grain is held withinsintered agglomerates; b) about 3-25 volume % bond; and c) about 35-80volume % total porosity, including at least 30 volume % interconnectedporosity, wherein pores included in the total porosity are irregularlyand randomly shaped and sized; the bonded abrasive tool beingsubstantially free of elongated shaped materials having a length tocross-sectional width aspect ratio of at least 5:1; wherein the sinteredagglomerates have an average diameter that is no greater than an averagedimension of the interconnected porosity when the interconnectedporosity is measured at a point of a maximum opening.
 23. The bondedabrasive tool of claim 22 wherein the bond is a vitrified bond and thetool has a maximum density of 2.2 g/cc.
 24. The bonded abrasive tool ofclaim 22 wherein the sintered agglomerates are 200 to 3,000 micrometersin average diameter.
 25. The bonded abrasive tool of claim 22 whereinthe abrasive grains are microabrasive grains and the sinteredagglomerates are 5 to 180 micrometers in average diameter.
 26. Thebonded abrasive tool of claim 22 wherein the bonded abrasive tool has abimodal porosity distribution of intra-agglomerate pores andinterconnected porosity.
 27. The bonded abrasive tool of claim 18wherein the bond is a vitrified bond and the tool has a maximum densityof 2.2 g/cc.
 28. The bonded abrasive toot of claim 18 wherein the toolfurther comprises at least one of secondary abrasive grain, fillermaterials, grinding aids, and pore inducing media.
 29. The bondedabrasive tool of claim 18 wherein the sintered agglomerates comprise aplurality of abrasive grains with a binding material, and the bindingmaterial has a melting temperature between about 500 and 1400° C. 30.The bonded abrasive tool of claim 18 wherein the sintered agglomeratescomprise a plurality of abrasive grains with a binding material and thebinding material has a melting temperature between about 500 and 1400°C., and the bond is a vitrified bond that has a firing temperature thatis lower than the melting temperature of the binding material.